[llvm] new-prs-labeler: Add `clang-tools-extra` labeling (PR #78633)

[Tom Stellard via llvm-commits]

https://github.com/tstellar approved this pull request.

LGTM. I've created the @llvm/pr-subscribers-clang-tools-extra team too.

https://github.com/llvm/llvm-project/pull/78633