Auto-draft PRs with fixup commits

[dmnks]

When making additional changes to PRs that are already under review, we typically resort to using git commit --fixup (or direct edits via the GH interface), as a way to allow the reviewer to clearly differentiate what's been added on top of the already reviewed content. They're also used by reviewers themselves when fixing typos and such. GH does show diffs between last pushes but the usability is not great, and it's GH-specific. Fixups are a much nicer, git-native way.

However, once pushing a fixup commit to a PR branch, GH won't prevent anybody from hitting merge, with those fixup commits ending up in mainline history, causing embarrassment. Therefore, the unwritten rule has been to turn such a PR to draft whenever making fixups. But that's a manual step that's easily forgotten...

So, add a GH action to automate this, by turning a PR to draft when there's a commit with a fixup! prefix pushed. Later, we could also automate the opposite ("undrafting") based on PR approval or something, but let's keep it simple for now.