Github Action: PR bot should merge changes in update branch

The TinyPilot PR bot will now automatically create PRs into the Pro repo any time we merge new changes into the Community repo (this repo). One slowdown in the workflow is that the branch it creates into Github Pro shows as out of date with Github Pro's master branch because of the hard reset in the Github Action.

The workaround is to click "Update branch" in the PR to merge the latest changes from the Pro's master branch. If there are no merge conflicts, you can do this entirely through the web UI.

It would be good if we could eliminate this friction by trying to merge automatically in the Github Action. This is a bit tricky because the bot can't resolve merge conflicts if they exist, but maybe the following workflow is possible:

1. Branch from Pro master branch
2. Pull from Community master branch
3. If there are merge conflicts abort the pull and hard reset to the Community master branch
4. Create the PR
5. If there were merge conflicts, developer manually resolves them during the PR

Start with .github/workflows/merge-pro.yml, especially the hard reset linked in the issue, and read the workflow steps that create the Pro pull request. Test the proposed branch-and-merge flow with both conflict-free and conflicting changes; done means the action incorporates Pro master when possible and still creates a PR for manual conflict resolution.