Add @propsbot merge action similar to Dependabot

[noisysocks]

Problem

It's cumbersome having to copy and paste the suggested props into the merge commit message.

Feature Request

If Propsbot gets the suggestions right, then I would like to be able to comment @propsbot merge and have it merge the PR and append all of the Co-authored-bys automatically.

We could have other commands too e.g. @propsbot refresh would be more ergonomic than adding a props-bot label to the PR.

This is inspired by Dependabot.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/managing-pull-requests-for-dependency-updates#managing-dependabot-pull-requests-with-comment-commands

Workaround

No response

Repository

https://github.com/WordPress/gutenberg