refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/mode-watcher.js

@@ -0,0 +1,21 @@
+/**
+ * Add listener for theme mode toggle
+ */
+const $toggleElem = $('.mode-toggle');
+
+export function modeWatcher() {
+  if ($toggleElem.length === 0) {
+    return;
+  }
+
+  $toggleElem.off().on('click', (e) => {
+    const $target = $(e.target);
+    let $btn =
+      $target.prop('tagName') === 'button'.toUpperCase()
+        ? $target
+        : $target.parent();
+
+    modeToggle.flipMode(); // modeToggle: `_includes/mode-toggle.html`
+    $btn.trigger('blur'); // remove the clicking outline
+  });
+}