targets/darwin: add more options for configuring macOS (#1753)

Add new options Darwin options:

- `targets.darwin.defaults`
  This adds options for configuring macOS through the `defaults(1)` system.
  This option can be used to manipulate a vast majority of user settings for macOS
  and its applications.

  This is implemented using freeform modules and includes additional descriptions
  and type information for some useful options.

- `targets.darwin.keybindings`
  This adds options for configuring the default keybindings for macOS text fields.

- `targets.darwin.search`
  This adds options for configuring the default search engine for macOS.

modules/misc/news.nix

@@ -1822,6 +1822,29 @@ in
           A new module is available: 'services.playerctld'.
         '';
       }
+
+      {
+        time = "2021-01-28T15:07:34+00:00";
+        condition = hostPlatform.isDarwin;
+        message = ''
+          New options are available for 'targets.darwin':
+
+          - targets.darwin.defaults
+
+            This adds options for configuring macOS through the defaults(1)
+            system.
+
+          - targets.darwin.keybindings
+
+            This adds options for configuring the default keybindings for macOS
+            text fields.
+
+          - targets.darwin.search
+
+            This adds options for configuring the default search engine for
+            macOS.
+        '';
+      }
     ];
   };
 }