More Prettier action fixing

[eemeli]

This shouldn't be as hard as it appears to be, but alas.

In forks where it's allowed to run, the action should still "just work".

For PRs coming from branches in this repo, the ref is now set so that the git push should "just work".

For PRs coming from branches in forks, it doesn't look like there's a way to get the action to have sufficient access so that it can write to that branch from the target repo, even if a human user could. So for those cases, the explicit repository is set and the action will only fail on git push if there are any files that Prettier should be run on.

In other words, the action should now work as a CI check when it's not able to automatically fix things.

A package.json is added, so that npm install && npm run prettier can be run locally to apply the same fixes as the action would.

The workflow_dispatch action will also allow triggering the action manually for a given branch.

CC @ryzokuken

[aphillips]

Is this ready? If so, let's merge it.