Avoid printing continuations within import identifiers

[charliermarsh]

Summary

It turns out that some identifiers can contain newlines -- specifically, dot-delimited import identifiers, like:

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 #7734.

Test Plan

cargo test

[codspeed-hq]

CodSpeed Performance Report

Merging #7744 will improve performances by 2.02%

_{Comparing charlie/continuation (a307da9) with main (d8a6279)}

Summary

⚡ 1 improvements
✅ 24 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	charlie/continuation	Change
⚡	linter/default-rules[numpy/ctypeslib.py]	19 ms	18.6 ms	+2.02%

[konstin]

I think that fundamentally that is bug in our AST: Indentifiers in python can not contain dots, and import foo.bar consists of two dot-separated keywords, not one.

I think we should merge this fix and then solve this properly with #7760