refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/post.js

@@ -0,0 +1,17 @@
+import { basic, initSidebar, initTopbar } from './modules/layouts';
+import {
+  imgExtra,
+  initLocaleDatetime,
+  initClipboard,
+  smoothScroll,
+  initPageviews
+} from './modules/plugins';
+
+basic();
+initSidebar();
+initTopbar();
+imgExtra();
+initLocaleDatetime();
+initClipboard();
+smoothScroll();
+initPageviews();