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

[Oliver Stöneberg via cfe-commits]

firewave wrote:

> Any thoughts on open-ended check name instead? `modernize-string-find-affix` (affix = prefix | suffix)?

`modernize-string-startswith-endswith` is the first what popped into my head but it would not have been my first choice.

Would this also be the check you would implement the suggested use of `contains()` in?

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