refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/categories.js

@@ -0,0 +1,7 @@
+import { basic, initSidebar, initTopbar } from './modules/layouts';
+import { categoryCollapse } from './modules/plugins';
+
+basic();
+initSidebar();
+initTopbar();
+categoryCollapse();