Add Pop!_OS

I’ve been using Cosmic as the default DE for about half a year now (after having used Gnome for over ten years) and it’s definitely my favorite these days. I can concur bugs keep popping up every now and then, but IMO it’s perfectly usable for tech enthousiast.

@Lukas Perhaps instead of Pop!_OS we can focus on Cosmic, since Cosmic can be used on arch based distros (e.g. I’m using it on top of Garuda).