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/builders.rs

@@ -0,0 +1,77 @@
+use ruff_formatter::prelude::*;
+use ruff_formatter::{write, Format};
+use ruff_text_size::TextRange;
+
+use crate::context::ASTFormatContext;
+use crate::core::types::Range;
+use crate::trivia::{Relationship, Trivia, TriviaKind};
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct Literal {
+    range: Range,
+}
+
+impl Format<ASTFormatContext<'_>> for Literal {
+    fn fmt(&self, f: &mut Formatter<ASTFormatContext<'_>>) -> FormatResult<()> {
+        let (text, start, end) = f.context().locator().slice(self.range);
+        f.write_element(FormatElement::StaticTextSlice {
+            text,
+            range: TextRange::new(start.try_into().unwrap(), end.try_into().unwrap()),
+        })
+    }
+}
+
+// TODO(charlie): We still can't use this everywhere we'd like. We need the AST
+// to include ranges for all `Ident` nodes.
+#[inline]
+pub const fn literal(range: Range) -> Literal {
+    Literal { range }
+}
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct LeadingComments<'a> {
+    comments: &'a [Trivia],
+}
+
+impl Format<ASTFormatContext<'_>> for LeadingComments<'_> {
+    fn fmt(&self, f: &mut Formatter<ASTFormatContext<'_>>) -> FormatResult<()> {
+        for comment in self.comments {
+            if matches!(comment.relationship, Relationship::Leading) {
+                if let TriviaKind::StandaloneComment(range) = comment.kind {
+                    write!(f, [hard_line_break()])?;
+                    write!(f, [literal(range)])?;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+#[inline]
+pub const fn leading_comments(comments: &[Trivia]) -> LeadingComments {
+    LeadingComments { comments }
+}
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct TrailingComments<'a> {
+    comments: &'a [Trivia],
+}
+
+impl Format<ASTFormatContext<'_>> for TrailingComments<'_> {
+    fn fmt(&self, f: &mut Formatter<ASTFormatContext<'_>>) -> FormatResult<()> {
+        for comment in self.comments {
+            if matches!(comment.relationship, Relationship::Trailing) {
+                if let TriviaKind::StandaloneComment(range) = comment.kind {
+                    write!(f, [hard_line_break()])?;
+                    write!(f, [literal(range)])?;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+#[inline]
+pub const fn trailing_comments(comments: &[Trivia]) -> TrailingComments {
+    TrailingComments { comments }
+}