Improved string processing POC

[MichaReiser]

Create a POC for #6936 to explore possible implementations and the performance implications.

[MichaReiser]

Learnings so far

• Black picks the preferred quote depending on the string parts that it merges. Meaning, the implicit string concatenation may have some parts that use single and other parts that use double quotes, depending on what requires the fewest escape for that specific line. Implementing this in Ruff is probably very hard, if not impossible. Ruff normalizes the entire string and picks a single quote for all parts
• I need to investigate if black applies the same formatting for triple quoted strings, and if it does, where ruff breaks these strings correctly. It could be a problem that the separator in 'flat' mode requires less space than the separator in expanded mode (one space vs triple quotes).