Remove exception-handler lexing from unused-bound-exception fix (#5851)

## Summary

The motivation here is that it will make this rule easier to rewrite as
a deferred check. Right now, we can't run this rule in the deferred
phase, because it depends on the `except_handler` to power its autofix.
Instead of lexing the `except_handler`, we can use the `SimpleTokenizer`
from the formatter, and just lex forwards and backwards.

For context, this rule detects the unused `e` in:

```python
try:
  pass
except ValueError as e:
  pass
```

crates/ruff_python_formatter/src/comments/format.rs

@@ -3,11 +3,11 @@ use rustpython_parser::ast::Ranged;
 
 use ruff_formatter::{format_args, write, FormatError, SourceCode};
 use ruff_python_ast::node::{AnyNodeRef, AstNode};
+use ruff_python_whitespace::{lines_after, lines_before, skip_trailing_trivia};
 
 use crate::comments::SourceComment;
 use crate::context::NodeLevel;
 use crate::prelude::*;
-use crate::trivia::{lines_after, lines_before, skip_trailing_trivia};
 
 /// Formats the leading comments of a node.
 pub(crate) fn leading_node_comments<T>(node: &T) -> FormatLeadingComments