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

[Oliver Stöneberg via cfe-commits]

firewave wrote:

I wonder if this should also detect the `str.compare("marker", 0, 6) == 0` pattern. There is possibly some kind of pattern involving `std::equal()` as well. Could as well be a different check though.

Not sure if it would have a performance impact to use `starts_with()` instead though.

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