refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/back-to-top.js

@@ -0,0 +1,26 @@
+/**
+ * Reference: https://bootsnipp.com/snippets/featured/link-to-top-page
+ */
+
+export function back2top() {
+  $(window).on('scroll', () => {
+    if (
+      $(window).scrollTop() > 50 &&
+      $('#sidebar-trigger').css('display') === 'none'
+    ) {
+      $('#back-to-top').fadeIn();
+    } else {
+      $('#back-to-top').fadeOut();
+    }
+  });
+
+  $('#back-to-top').on('click', () => {
+    $('body,html').animate(
+      {
+        scrollTop: 0
+      },
+      800
+    );
+    return false;
+  });
+}