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`

crates/ruff_python_formatter/tests/snapshots/format@statement__import.py.snap

@@ -8,6 +8,13 @@ from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjf
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as dfgsdfgsd, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd
 
+# Continuations.
+import foo\
+    .bar
+
+from foo\
+    .bar import baz
+
 # At the top-level, force one empty line after an import, but allow up to two empty
 # lines.
 import os
@@ -98,6 +105,11 @@ from a import (
     aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd,
 )
 
+# Continuations.
+import foo.bar
+
+from foo.bar import baz
+
 # At the top-level, force one empty line after an import, but allow up to two empty
 # lines.
 import os