Add a networking.wireless module

[sellout]

Ideally compatible with the one in NixOS, I’m looking to make my networks declarative and shared between OSes.

This should be doable via the networksetup command.

Possibly include an option to remove existing networks first (alternatively, keep track of which ones were defined declaratively, so they they can be deleted when removed from the config).

I’m inclined to do this for myself anyway, but seeing if there’s interesti in polishing it enough for upstream, and any suggestions on exactly what to do. It definitely needs to support secrets, like networking.wireless.environmentFile does, but I’m open to doing things differently than NixOS.