[clang-tools-extra] [clang-tidy] Add new modernize-string-find-startswith check (PR #72385)

[Nicolas van Kempen via cfe-commits]

nicovank wrote:

I like `XXX-use-starts-ends-with`. I would rather move it to `performance` than `readability` if not `modernize`. @PiotrZSL thoughts before I go ahead and do the renaming? Also move it somewhere else?

https://github.com/llvm/llvm-project/pull/72385