move options back out to their config files, improve documentation

modules/features/shells.nix

@@ -1,10 +1,16 @@
 { inputs, self, ... }: {
+  imports = [ (inputs.helpers.mkMirroredModulesFlakeModule "globalOptions" ({ lib, ... }: {
+    options.features.shells.defaultShell = lib.mkOption {
+      type = inputs.helpers.mkEnumFromDirContents "${self}/modules/features/shells";
+      description = "This option controls the shell that should be used as the default, enabling that shell in the process.";
+      default = "zsh";
+    };
+  })) ];
+
   flake.nixosModules.global = { config, pkgs, lib, ... }: {
     programs.${config.features.shells.defaultShell}.enable = true;
 
     users.defaultUserShell = pkgs.${config.features.shells.defaultShell};
     users.users."grey".shell = pkgs.${config.features.shells.defaultShell};
-
-    features.shells.${config.features.shells.defaultShell}.enable = true;
   };
 }