Code that was in `/clock` has been moved to github.com/coder/quartz. This PR refactors our use of the clock library to point to the external Quartz repo.
Refactor autobuild/notify and tests to use the clock testing library.
I also rewrote some of the comments because I didn't understand them when I was looking at the package.
* feat: send native system notification on scheduled workspace shutdown
This commit adds a fairly generic notification package and uses it
to notify users connected over SSH of pending workspace shutdowns.
Only one notification will be sent at most 5 minutes prior to the scheduled
shutdown, and only one CLI instance will send notifications if multiple
instances are running.
