meta: Replace zellij with tmux

Now that tmux works again, it makes sense to choose it over zellij due
to the vastly superior community support around it. Using tmux-256color
makes colors work properly in programs like htop, and neovim benefits
from squiggly lines and italics from kitty as well.

modules/shell.nix

@@ -71,7 +71,6 @@ in
         inherit (pkgs)
           gdu
           hexyl
-          zellij
           diskonaut
           pgcli
           litecli