add action to check project changes and provide feedback on each PR

[shiftkey]

This PR adds an action that runs on every pull_request event and does a few things:

• check the PR adds or modifies any files under _data/projects
• parses the files and checks they are valid
• for GitHub projects, run additional checks that
  • the repository exists and is valid
  • the label specified is in use
  • the label and link are in sync when the project links directly to the label

This is codifying several checks that I've been manually doing up until now, and it'd be great to not have to do this to save time. Ideally this comment will help contributors fix things without needing to dig into the Actions logs to see why something failed...

[shiftkey]

I've been poking at this locally with some contrived scenarios, but I'm going to merge this in next and see if I can run it against some open PRs and get some real-world testing out of it.