proposal: adopt precommit.ci

[drammock]

It's a CI action that runs our pre-commit hooks in a CI action, and can be configured to auto-fix some failures. https://pre-commit.ci/

Would have saved some effort in #1711, for example. @trallard is in favor (see #1711 (comment)), and I'm on board too (we use it in other projects I work on).

Any objections? The main downside is that for users not super familiar with git, having a bot add commits to your PR can potentially be a bit confusing / cause subsequent pushes to fail until you've done git pull --rebase or similar. But IMO git's error messages are quite clear/instructive in that particular case, and in my experience the bot solves more problems than it creates.