Adopt `actions/labeler` Action

[yoshuawuyts]

https://github.com/actions/labeler looks particularly useful, enabling us to automatically label PRs based on which files they are made against. This seems like it might compliment the work on code owners well #829, which will handle automatic issue assignments.

With this the only piece missing would be "subscribe to label", which is currently behind a GitHub feature flag. But I'll see what I can do on my end to enable that.