demos: Add setup for plain js demos (#2288)

* add basic js demo

* improve js demos

* change demo button

* fix js demos on build

* add tailwind experiment

* docs: replace tailwind demo

Co-authored-by: Philipp Kühn <philippkuehn@MacBook-Pro-von-Philipp.local>

demos/preview/Demo.vue

@@ -114,7 +114,7 @@ export default {
       sources: {},
       currentTab: null,
       currentFile: null,
-      tabOrder: ['React', 'Vue'],
+      tabOrder: ['React', 'Vue', 'JS'],
       debugJSON: null,
       showDebug: false,
     }