[clang-tools-extra] [clang-tidy] Add readability-string-view-substr check (PR #120055)

[Helmut Januschka via cfe-commits]

hjanuschka wrote:

@5chmidti may i ping you? could you help me out, it feels like its only inches away from being done. or could we go with the prev. commit as mentioned above?

https://github.com/llvm/llvm-project/pull/120055