extensions-tool: Prepare for alternative templates

The template used when creating a new extension is intentionally
minimal, as the sample code in the old extensions-tool proved to
be annoying more often than not.

However as we support per-command options, we don't have to limit
ourselves to a single template, and can offer alternatives for
common use cases.

To prepare for that, namespace the existing template by moving it
into a subfolder.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/812

subprojects/extensions-tool/src/templates/plain/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();
+}

subprojects/extensions-tool/src/templates/plain/stylesheet.css

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