This adds a Boolean option `uninstall`. When enabled this option will
reset side-effecting configurations to their "empty" state. The intent
is that this will cause the activation script to remove all managed
files and packages.
Doing it this way should hopefully be more robust than the previous
solution. It also allows a somewhat more convenient uninstall process
when using Flakes; put `uninstall = true` in your existing
configuration and then do a switch.
Also add simple uninstall test in CI test job.
