refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/page.js

@@ -0,0 +1,9 @@
+import { basic, initSidebar, initTopbar } from './modules/layouts';
+import { imgExtra, initClipboard, smoothScroll } from './modules/plugins';
+
+basic();
+initSidebar();
+initTopbar();
+imgExtra();
+initClipboard();
+smoothScroll();