Ignore end-of-line file exemption comments (#6160)
## Summary
This PR protects against code like:
```python
from typing import Optional
import bar # ruff: noqa
import baz
class Foo:
x: Optional[str] = None
```
In which the user wrote `# ruff: noqa` to ignore a specific error, not
realizing that it was a file-level exemption that thus turned off all
lint rules.
Specifically, if a `# ruff: noqa` directive is not at the start of a
line, we now ignore it and warn, since this is almost certainly a
mistake.
