Ideally, this would be on "labeled" types of pull request events, but that doesn't work if the pull request is from another fork. For example, see https://github.com/actions/labeler/issues/12
This allows users to follow certain labels and automatically get @-mentioned when they are applied to an issue or a pull request. See https://github.com/bytecodealliance/subscribe-to-label-action for details. Fixes #1234