fix: don't try to format strings inside f-strings

[tusharsadhwani]

Resolves #4350 , closes #4365

If a string like '\'' is inside a double-quoted f-string, like f"{'\''}" the formatter would break trying to remove the escaped \' quote.

This is because normalize_string_quotes looks at the entire f"{'\''}" input, and the outermost quotes ", hence determining that the \' escape is unnecessary.

To avoid these cases, we don't try to normalize inner strings inside f-string braces.

[github-actions]

diff-shades reports zero changes comparing this PR (b52892e) to main (7e2afc9).

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[hauntsaninja]

Thanks, looks like an actually correct version of #4365 :-)

This has an amusing interaction with the stability policy, but better to have not the final formatting we want than have a crash