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

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

@@ -0,0 +1,19 @@
+/**
+ * Reference: https://bootsnipp.com/snippets/featured/link-to-top-page
+ */
+
+export function back2top() {
+  const btn = document.getElementById('back-to-top');
+
+  window.addEventListener('scroll', () => {
+    if (window.scrollY > 50) {
+      btn.classList.add('show');
+    } else {
+      btn.classList.remove('show');
+    }
+  });
+
+  btn.addEventListener('click', () => {
+    window.scrollTo({ top: 0 });
+  });
+}