## Summary
The motivation here is that this enables us to implement `Ranged` in
crates that don't depend on `ruff_python_ast`.
Largely a mechanical refactor with a lot of regex, Clippy help, and
manual fixups.
## Test Plan
`cargo test`
## Summary
Previously, given:
```python
a = \
5;
```
When detecting continuations starting at the offset of the `;`, we'd
flag the previous line as a continuation. We should only flag a
continuation if there isn't leading content prior to the offset.
Closes https://github.com/astral-sh/ruff/issues/6214
