Create GitHub workflow to remind developers to update changelogs

[mcmire]

We want to ensure that going forward, contributors update changelogs as they make changes and don't wait to do this until the release PR.

Essentially this is a clone of this ticket but suited specifically for the core repo.

Copying what Mark wrote there:

The workflow could look something like this:

• When a PR is marked as "Ready for review", check if there are changelog modifications

• If there are no modifications, leave a comment (once, per PR) asking the contributor to document changes, or to indicate there are no changes

  • This indication could be via a "no-changes" label, or a comment like @metamask no-changes, something like that.

Additionally we can have a PR status check to ensure this was followed.

• If there are changelog modifications OR an indication of no changes, the status check should pass.
• If there are no changelog modifications and no indication of no changes, the status check should be "pending".

This would ensure that PRs are blocked on changes being documented, or on there being an explicit indication that there are no changes. It should be easy to understand and follow for contributors. And using "pending" rather than "fail" for the status check ensures it doesn't give a false impression of the branch being broken.