Extract string part and normalized string

[MichaReiser]

Summary

This is a small refactor that extracts a NormalizedString data structure rather than just doing normalization during formatting.

Having a dedicated data structure that caches the normalized strings is important for implementing Black's preview style string formatting where it is necessary
to format the normalized string multiple times:

1. Join all parts together
2. Using the same separation as in the source
3. Try to fit as many words as possible

Test Plan

cargo test

[MichaReiser]

Current dependencies on/for this PR:

• main
  • PR Split implicit concatenated strings before binary expressions #7145
    • PR Formatter: Implicit concatenation in compare expressions #7193
      • PR Add FString support to binary like formatting #7215
        • PR Add PreviewMode option to formatter #7217
          • PR Extract string part and normalized string #7219 👈
            • PR Introduce StringContinuation data structure #7242
              • PR POC of Black's string preview style formatting #7567

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