meta: Begin making shell module customizable

modules/shell.nix

@@ -1,9 +1,16 @@
-{ pkgs, ... }:
+{ pkgs, config, lib, ... }:
 
 let
   inherit (pkgs) fish htop-vim;
+  inherit (lib) mkEnableOption mkIf;
+
+  cfg = config.modules.shell;
 in
 {
+  options.modules.shell = {
+  };
+
+  config = with cfg; {
     users.defaultUserShell = fish;
     environment.shells = [ fish ];
 
@@ -275,4 +282,5 @@ in
         };
       };
     };
+  };
 }