Attempt to automatically keep PRs updated

[Jake-Shadle]

It's useful to keep a PR updated with the branch it is going to be merged into, Github supports merging from the base branch as a preview feature, but doesn't support rebasing, so the easiest would just be to support both in the bot. This could either use the CLI or libgit2, though I would lean towards the CLI since it would 0 extra compile time/binary size for something that is (probably) available in the action environment.

[bnjbvr]

+1 to this; maybe a slightly different, simpler feature would be to try to rebase once the PR has been approved, just before a merge? (And comment if automatic rebase can't be done.)
This would avoid merge skew (see also) and make sure that green CI on PR implies the CI is green on main too.