Add initial formatter implementation (#2883)

# Summary

This PR contains the code for the autoformatter proof-of-concept.

## Crate structure

The primary formatting hook is the `fmt` function in `crates/ruff_python_formatter/src/lib.rs`.

The current formatter approach is outlined in `crates/ruff_python_formatter/src/lib.rs`, and is structured as follows:

- Tokenize the code using the RustPython lexer.
- In `crates/ruff_python_formatter/src/trivia.rs`, extract a variety of trivia tokens from the token stream. These include comments, trailing commas, and empty lines.
- Generate the AST via the RustPython parser.
- In `crates/ruff_python_formatter/src/cst.rs`, convert the AST to a CST structure. As of now, the CST is nearly identical to the AST, except that every node gets a `trivia` vector. But we might want to modify it further.
- In `crates/ruff_python_formatter/src/attachment.rs`, attach each trivia token to the corresponding CST node. The logic for this is mostly in `decorate_trivia` and is ported almost directly from Prettier (given each token, find its preceding, following, and enclosing nodes, then attach the token to the appropriate node in a second pass).
- In `crates/ruff_python_formatter/src/newlines.rs`, normalize newlines to match Black’s preferences. This involves traversing the CST and inserting or removing `TriviaToken` values as we go.
- Call `format!` on the CST, which delegates to type-specific formatter implementations (e.g., `crates/ruff_python_formatter/src/format/stmt.rs` for `Stmt` nodes, and similar for `Expr` nodes; the others are trivial). Those type-specific implementations delegate to kind-specific functions (e.g., `format_func_def`).

## Testing and iteration

The formatter is being developed against the Black test suite, which was copied over in-full to `crates/ruff_python_formatter/resources/test/fixtures/black`.

The Black fixtures had to be modified to create `[insta](https://github.com/mitsuhiko/insta)`-compatible snapshots, which now exist in the repo.

My approach thus far has been to try and improve coverage by tackling fixtures one-by-one.

## What works, and what doesn’t

- *Most* nodes are supported at a basic level (though there are a few stragglers at time of writing, like `StmtKind::Try`).
- Newlines are properly preserved in most cases.
- Magic trailing commas are properly preserved in some (but not all) cases.
- Trivial leading and trailing standalone comments mostly work (although maybe not at the end of a file).
- Inline comments, and comments within expressions, often don’t work -- they work in a few cases, but it’s one-off right now. (We’re probably associating them with the “right” nodes more often than we are actually rendering them in the right place.)
- We don’t properly normalize string quotes. (At present, we just repeat any constants verbatim.)
- We’re mishandling a bunch of wrapping cases (if we treat Black as the reference implementation). Here are a few examples (demonstrating Black's stable behavior):

```py
# In some cases, if the end expression is "self-closing" (functions,
# lists, dictionaries, sets, subscript accesses, and any length-two
# boolean operations that end in these elments), Black
# will wrap like this...
if some_expression and f(
    b,
    c,
    d,
):
    pass

# ...whereas we do this:
if (
    some_expression
    and f(
        b,
        c,
        d,
    )
):
    pass

# If function arguments can fit on a single line, then Black will
# format them like this, rather than exploding them vertically.
if f(
    a, b, c, d, e, f, g, ...
):
    pass
```

- We don’t properly preserve parentheses in all cases. Black preserves parentheses in some but not all cases.

crates/ruff_python_formatter/src/format/arguments.rs

@@ -0,0 +1,123 @@
+use ruff_formatter::prelude::*;
+use ruff_formatter::{format_args, write, Format};
+
+use crate::context::ASTFormatContext;
+use crate::cst::Arguments;
+use crate::shared_traits::AsFormat;
+
+pub struct FormatArguments<'a> {
+    item: &'a Arguments,
+}
+
+impl AsFormat<ASTFormatContext<'_>> for Arguments {
+    type Format<'a> = FormatArguments<'a>;
+
+    fn format(&self) -> Self::Format<'_> {
+        FormatArguments { item: self }
+    }
+}
+
+impl Format<ASTFormatContext<'_>> for FormatArguments<'_> {
+    fn fmt(&self, f: &mut Formatter<ASTFormatContext<'_>>) -> FormatResult<()> {
+        let args = self.item;
+
+        let mut first = true;
+
+        let defaults_start = args.posonlyargs.len() + args.args.len() - args.defaults.len();
+        for (i, arg) in args.posonlyargs.iter().chain(&args.args).enumerate() {
+            if !std::mem::take(&mut first) {
+                write!(f, [text(",")])?;
+                write!(f, [soft_line_break_or_space()])?;
+            }
+
+            write!(
+                f,
+                [group(&format_args![format_with(|f| {
+                    write!(f, [arg.format()])?;
+                    if let Some(i) = i.checked_sub(defaults_start) {
+                        if arg.node.annotation.is_some() {
+                            write!(f, [space()])?;
+                            write!(f, [text("=")])?;
+                            write!(f, [space()])?;
+                        } else {
+                            write!(f, [text("=")])?;
+                        }
+                        write!(f, [args.defaults[i].format()])?;
+                    }
+                    Ok(())
+                })])]
+            )?;
+
+            if i + 1 == args.posonlyargs.len() {
+                if !std::mem::take(&mut first) {
+                    write!(f, [text(",")])?;
+                    write!(f, [soft_line_break_or_space()])?;
+                }
+                write!(f, [text("/")])?;
+            }
+        }
+
+        if let Some(vararg) = &args.vararg {
+            if !std::mem::take(&mut first) {
+                write!(f, [text(",")])?;
+                write!(f, [soft_line_break_or_space()])?;
+            }
+            first = false;
+
+            write!(f, [text("*")])?;
+            write!(f, [vararg.format()])?;
+        } else if !args.kwonlyargs.is_empty() {
+            if !std::mem::take(&mut first) {
+                write!(f, [text(",")])?;
+                write!(f, [soft_line_break_or_space()])?;
+            }
+            first = false;
+
+            write!(f, [text("*")])?;
+        }
+
+        let defaults_start = args.kwonlyargs.len() - args.kw_defaults.len();
+        for (i, kwarg) in args.kwonlyargs.iter().enumerate() {
+            if !std::mem::take(&mut first) {
+                write!(f, [text(",")])?;
+                write!(f, [soft_line_break_or_space()])?;
+            }
+
+            write!(
+                f,
+                [group(&format_args![format_with(|f| {
+                    write!(f, [kwarg.format()])?;
+                    if let Some(default) = i
+                        .checked_sub(defaults_start)
+                        .and_then(|i| args.kw_defaults.get(i))
+                    {
+                        if kwarg.node.annotation.is_some() {
+                            write!(f, [space()])?;
+                            write!(f, [text("=")])?;
+                            write!(f, [space()])?;
+                        } else {
+                            write!(f, [text("=")])?;
+                        }
+                        write!(f, [default.format()])?;
+                    }
+                    Ok(())
+                })])]
+            )?;
+        }
+        if let Some(kwarg) = &args.kwarg {
+            if !std::mem::take(&mut first) {
+                write!(f, [text(",")])?;
+                write!(f, [soft_line_break_or_space()])?;
+            }
+
+            write!(f, [text("**")])?;
+            write!(f, [kwarg.format()])?;
+        }
+
+        if !first {
+            write!(f, [if_group_breaks(&text(","))])?;
+        }
+
+        Ok(())
+    }
+}