[clang-tidy] Add abseil-string-find-str-contains checker.

Summary: This adds a checker which suggests replacing string.find(...) == npos with absl::StrContains.

Reviewers: alexfh, hokein, aaron.ballman, njames93, ymandel

Reviewed By: ymandel

Subscribers: ymandel, Eugene.Zelenko, xazax.hun, mgorny, Charusso, phosek, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D80023