Automate GitHub releases

[lorenzwalthert]

• could use a post merge hook or GitHub Action when PR from hook-dependency-update was merged into main. But probably easier is just as part of the hook dependency update. Also belongs together semantically.
• release_gh() works locally currently.
• Deals with {git2r} for committing things, although the create-pr action could take care of that.
• tags must be on main branch to be recognised with pre-commit autoupdate. Does it work if we tag on branch hook-dependency-update and then merge to main?