fix: Stop moving multiline strings to a new line unless inside brackets

[cobaltt7]

Description

Resolves #4247

I fixed it by not wrapping around multiline strings outside brackets. This keeps all existing tests passing while allowing assert statements to not be reformatted. This is currently gated under the multiline_string_handling unstable style, since it's the smallest solution to just add it in there.

Checklist - did you ...

• [y] Add an entry in CHANGES.md if necessary?
• [y] Add / update tests if necessary?
• [-] Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (001ab8f) to main (c9d2635).

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[hauntsaninja]

Thanks you!