[red-knot] add maybe-undefined lint rule (#12414)

Add a lint rule to detect if a name is definitely or possibly undefined
at a given usage.

If I create the file `undef/main.py` with contents:

```python
x = int
def foo():
    z
    return x
if flag:
    y = x
y
```

And then run `cargo run --bin red_knot -- --current-directory
../ruff-examples/undef`, I get the output:

```
Name 'z' used when not defined.
Name 'flag' used when not defined.
Name 'y' used when possibly not defined.
```

If I modify the file to add `y = 0` at the top, red-knot re-checks it
and I get the new output:

```
Name 'z' used when not defined.
Name 'flag' used when not defined.
```

Note that `int` is not flagged, since it's a builtin, and `return x` in
the function scope is not flagged, since it refers to the global `x`.

crates/ruff_db/Cargo.toml

@@ -15,6 +15,7 @@ ruff_cache = { workspace = true, optional = true }
 ruff_notebook = { workspace = true }
 ruff_python_ast = { workspace = true }
 ruff_python_parser = { workspace = true }
+ruff_python_trivia = { workspace = true }
 ruff_source_file = { workspace = true }
 ruff_text_size = { workspace = true }
 

crates/ruff_db/src/system/test.rs

@@ -1,4 +1,5 @@
 use ruff_notebook::{Notebook, NotebookError};
+use ruff_python_trivia::textwrap;
 
 use crate::files::File;
 use crate::system::{DirectoryEntry, MemoryFileSystem, Metadata, Result, System, SystemPath};
@@ -150,6 +151,12 @@ pub trait DbWithTestSystem: Db + Sized {
         result
     }
 
+    /// Writes auto-dedented text to a file.
+    fn write_dedented(&mut self, path: &str, content: &str) -> crate::system::Result<()> {
+        self.write_file(path, textwrap::dedent(content))?;
+        Ok(())
+    }
+
     /// Writes the content of the given files and notifies the Db about the change.
     ///
     /// # Panics