Add a GitHub Action for triaging BCR PR review (#1865)

With this GitHub action, BCR modules maintainers will be automatically
pinged on PRs that modify their maintained modules. If no maintainers
are found, then fall back to notifying the registry maintainers.

Tested at 
- https://github.com/meteorcloudy/bazel-central-registry/pull/11
- https://github.com/meteorcloudy/bazel-central-registry/pull/12

Working towards:
https://github.com/bazelbuild/bazel-central-registry/issues/130
530 files changed