wip(js): add head.html and assets/js

assets/js/modules/components/mode-toggle.js

@@ -0,0 +1,15 @@
+/**
+ * Add listener for theme mode toggle
+ */
+
+const $toggle = document.getElementById('mode-toggle');
+
+export function modeWatcher() {
+  if (!$toggle) {
+    return;
+  }
+
+  $toggle.addEventListener('click', () => {
+    Theme.flip();
+  });
+}