Introduce StringContinuation data structure

[MichaReiser]

Summary

Similar to the preceding PR. It extracts a StringContinuation data structure that allows inspection.
Being able to access the continuation parts quickly is important to implement Black's string preview style where we need create three different variations for each implicit concatenated string.

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

This comment was auto-generated by Graphite.

[codspeed-hq]

CodSpeed Performance Report

Merging #7242 will degrade performances by 5.44%

_{Comparing create-string-continuation (272306b) with main (0a07a2c)}

Summary

⚡ 6 improvements
❌ 5 regressions
✅ 14 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	create-string-continuation	Change
⚡	formatter[pydantic/types.py]	19.9 ms	19.1 ms	+4.27%
❌	linter/all-rules[numpy/globals.py]	4 ms	4.1 ms	-2.77%
⚡	formatter[large/dataset.py]	57.7 ms	52.3 ms	+10.29%
❌	linter/all-rules[unicode/pypinyin.py]	15.4 ms	15.7 ms	-2.22%
⚡	formatter[unicode/pypinyin.py]	3.6 ms	3.4 ms	+7.85%
⚡	formatter[numpy/ctypeslib.py]	10.9 ms	9.9 ms	+9.29%
❌	linter/all-rules[pydantic/types.py]	70 ms	72.9 ms	-4.04%
⚡	lexer[large/dataset.py]	10.1 ms	9.8 ms	+3.25%
❌	linter/all-rules[numpy/ctypeslib.py]	33.2 ms	34.2 ms	-3.01%
⚡	formatter[numpy/globals.py]	1.1 ms	1.1 ms	+2.35%
❌	linter/all-rules[large/dataset.py]	155.1 ms	164 ms	-5.44%