Lint PR titles with GitHub action

[msrose]

The PR title is the commit message that eventually gets added to main, so it would be handy to lint for a conventional-commits prefix with a GitHub action since we don't have any way to enforce it right now if the author edits the title manually. There are some existing actions I've seen that we could maybe use, but did not seem to suit our exact needs at the time -- still worth checking out again before writing our own.