Allows setting home.shellAbbrs that either get set as Fish shell
abbreviations or converted to shell aliases depending on if Fish is
active.
Abbrs that depend on Fish-specific features get automatically filtered
out.
Instead of having separate folders for each program with a default.nix
inside, moved modules into programs/services/xorg respectively. Xdg was
moved to the root home folder as it makes the most sense there.
