Refactor: Remove StringPart and AnyStringPart in favor of StringLikePart

[MichaReiser]

Summary

I don't think having both types still makes sense.
The way I remember it is that we had StringPart first, a generic representation of any string part.
We later introduced AnyStringPart without realizing that we now have two generic representations of a string part. We then also added StringLike when we needed a similar construct in the linter. And there were three representations :)

This PR removes StringPart and AnyStringPart and replaces them with StringLikePart, to only have one generic string part representation.

Split out from #13663

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dhruvmanila]

This is great, lots of code removed. Thanks for doing this!

I agree that it's a bit annoying to add formatter specific methods but the StringLikeExtensions trait is a good idea.