Avoid printing continuations within import identifiers (#7744)
## Summary
It turns out that _some_ identifiers can contain newlines --
specifically, dot-delimited import identifiers, like:
```python
import foo\
.bar
```
At present, we print all identifiers verbatim, which causes us to retain
the `\` in the formatted output. This also leads to violating some debug
assertions (see the linked issue, though that's a symptom of this
formatting failure).
This PR adds detection for import identifiers that contain newlines, and
formats them via `text` (slow) rather than `source_code_slice` (fast) in
those cases.
Closes https://github.com/astral-sh/ruff/issues/7734.
## Test Plan
`cargo test`
