Introduce `gdk kill` command

Overview

On occasion, some of the GDK services do not stop correctly. The specific services and frequency vary wildly but it happens frequently enough that it's disruptive to productivity.

Let's introduce a gdk kill command that will perform a 'hard kill' using pkill -9 runsv.

We'll need to provide a warning that this command will kill all runsv processes, which may include other runit services including potentially other running GDKs which is why we can't automatically run. If the warning is clear enough then we leave this decision up to the user which I feel is a fair trade off.