Add command/workflow to go from S-waiting-on-author to S-waiting-on-review

[XAMPPRocky]

Currently the status labels for PRs are mostly manually updated by triagers, it would be nice if authors of PRs could more easily update the status when it's S-waiting-on-author when they believe they have completed the necessary changes.

Potential Solutions

• Detect when the author has requested a review using GitHub's request review feature and automatically update labels.
• Add some kind of @rustbot review command.

I think ideally we would have both, as @rustbot review is easy to teach, but more people are using GitHub's review features as it better integrates with GitHub notifications, and the @rustbot review command could be implemented so that it uses GitHub's request review feature.

[kellda]

I think it has been fixed by #1381...