run hooks in the background with a runner daemon

[jmelahman]

I was looking to see if jj had hook support and stumbled across this comment, jj-vcs/jj#3577 (comment)

The idea is run a daemon in the background (prek serve, maybe runs as a systemd unit) with a filewatcher that runs the hooks periodically or as files change and caches their results. On commit/push, the results are brought to the foreground, resuming progress if they were unfinished.