add features.applications.shellScripts option

uses a custom function from my helper flake to convert a basic attrset of strings into a format `home.file` will accept

modules/features/shells.nix

@@ -0,0 +1,17 @@
+{ inputs, self, ... }: {
+  flake.nixosModules.global = { config, pkgs, lib, ... }: {
+    options.features.shells.defaultShell = lib.mkOption {
+      type = lib.types.enum (map (name: lib.removeSuffix ".nix" name) (
+        builtins.attrNames (builtins.readDir "${self}/modules/features/shells")
+      ));
+      default = "zsh";
+    };
+
+    config = {
+      programs.${config.features.shells.defaultShell}.enable = true;
+
+      users.defaultUserShell = pkgs.${config.features.shells.defaultShell};
+      users.users."grey".shell = pkgs.${config.features.shells.defaultShell};
+    };
+  };
+}