extensions-tool: Implement create command

This implements more functionality of the existing tool and, as
'reload' is an unreliable feature that doesn't work more often
than not, the last bit that we will replicate.

The command follows the original for the most part, with the most
important difference being the installed template, which doesn't
provide any sample functionality and uses modern JS syntax.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1234

src/extensions-tool/template/extension.js

@@ -0,0 +1,34 @@
+/* extension.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
+ */
+
+/* exported init */
+
+class Extension {
+    constructor() {
+    }
+
+    enable() {
+    }
+
+    disable() {
+    }
+}
+
+function init() {
+    return new Extension();
+}

src/extensions-tool/template/stylesheet.css

@@ -0,0 +1 @@
+/* Add your custom extension styling here */