Optimize code. Update templates.

src/components/Playground/templates/demo/main.tsx

@@ -0,0 +1,10 @@
+import React from 'react'
+import ReactDOM from 'react-dom/client'
+
+import App from './App'
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+    <React.StrictMode>
+        <App />
+    </React.StrictMode>
+)