Sync tags on matching paths

[ppaskaris-coursera]

It would be neat to have an action that applies or removes tags from a PR depending on which paths are affected in the diff. This would enable someone like myself to bookmark a PR search by tag to separate PRs that I care about for a particular facet from the rest of the monorepo.

To identify a situation where a tag should be applied, I would like to test the modified paths against a regular expression. If the PR has a matching path, apply the tag, otherwise, remove the tag if it's already present. Use case for removing is if someone reverts a change, and the PR no longer affects what I care about, I would not like to see it.

This is similar to how path-based Herald subscriptions work, but without automagically sending you notifications, just letting you filter by topic (tag, replacement for group subscription).