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_whitespace/src/lib.rs

@@ -1,7 +1,9 @@
 mod cursor;
 mod newlines;
+mod tokenizer;
 mod whitespace;
 
 pub use cursor::*;
 pub use newlines::*;
+pub use tokenizer::*;
 pub use whitespace::*;