tidy: add triagebot checks Validates triagebot.toml to have existing paths: `[mentions."*"]` sections, i.e. ```toml [mentions."compiler/rustc_const_eval/src/"] ``` or ```toml [assign.owners] "/.github/workflows" = ["infra-ci"] ``` or ```toml trigger_files = [ "src/librustdoc/html/static/js/search.js", "tests/rustdoc-js", "tests/rustdoc-js-std", ] ``` Looked at #137876 and implemented check.