common-workstation: Add zsh configuration.

I'm tired of copying around my zsh configuration file, which I didn't
include in my NixOS configurations because it was quite messy so far.

It is still a bit messy, but at least most of the old crap is gone now,
and we're using programs.zsh to configure it.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>