refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/category-collapse.js

@@ -0,0 +1,36 @@
+/**
+ * Tab 'Categories' expand/close effect.
+ */
+const childPrefix = 'l_';
+const parentPrefix = 'h_';
+const collapse = $('.collapse');
+
+export function categoryCollapse() {
+  /* close up top-category */
+  collapse.on('hide.bs.collapse', function () {
+    /* Bootstrap collapse events. */ const parentId =
+      parentPrefix + $(this).attr('id').substring(childPrefix.length);
+    if (parentId) {
+      $(`#${parentId} .far.fa-folder-open`).attr(
+        'class',
+        'far fa-folder fa-fw'
+      );
+      $(`#${parentId} i.fas`).addClass('rotate');
+      $(`#${parentId}`).removeClass('hide-border-bottom');
+    }
+  });
+
+  /* expand the top category */
+  collapse.on('show.bs.collapse', function () {
+    const parentId =
+      parentPrefix + $(this).attr('id').substring(childPrefix.length);
+    if (parentId) {
+      $(`#${parentId} .far.fa-folder`).attr(
+        'class',
+        'far fa-folder-open fa-fw'
+      );
+      $(`#${parentId} i.fas`).removeClass('rotate');
+      $(`#${parentId}`).addClass('hide-border-bottom');
+    }
+  });
+}