Change app component declaration from arrow function to regular function (#6655)

* Change app component declaration from arrow function to regular function

* Change TypeScript template back to an arrow function

Author: iansu

packages/react-scripts/template/src/App.js

@@ -2,7 +2,7 @@ import React from 'react';
 import logo from './logo.svg';
 import './App.css';
 
-const App = () => {
+function App() {
   return (
     <div className="App">
       <header className="App-header">