split string module

[MichaReiser]

Summary

I started to find it difficult to navigate the string/mod.rs and decided to split it into any containing the AnyString* types
and normalize containing the normalization logic.

This has the benefit that mod.rs now mainly contains the code related to formating continuations, quotes, and prefixes.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dhruvmanila]

Makes sense 👍