[clang-tools-extra] [clang-tidy] Add type annotations to add_new_check.py, fix minor bug (PR #106801)

[Nicolas van Kempen via cfe-commits]

nicovank wrote:

Thanks for review! Addressed comments.

I have no commit access, if/when this looks good to go you can go ahead and click merge.

https://github.com/llvm/llvm-project/pull/106801