extensions-tool/create: Optionally include prefs

We currently only create boilerplate code for the actual
extension. Now that libadwaita has largely standardized
preference UIs, it makes sense to allow creating the
prefs.js boilerplate as well.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2889>

subprojects/extensions-tool/src/templates/prefs.js

@@ -0,0 +1,44 @@
+/* prefs.js
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+import Adw from 'gi://Adw';
+import GObject from 'gi://GObject';
+
+import {ExtensionPreferences, gettext as _} from 'resource:///org/gnome/Shell/Extensions/js/extensions/prefs.js';
+
+const MyPreferencesWidget = GObject.registerClass(
+class MyPreferencesWidget extends Adw.PreferencesGroup {
+    constructor() {
+        super({
+            title: _('Section'),
+        });
+
+        this.add(new Adw.SwitchRow({
+            title: _('Title'),
+            subtitle: _('Subtitle'),
+            active: true,
+        }));
+    }
+});
+
+export default class MyPreferences extends ExtensionPreferences {
+    getPreferencesWidget() {
+        return new MyPreferencesWidget();
+    }
+}
+