refactor(examples): switch examples from cra to nextjs

examples/nextjs/pages/_app.tsx

@@ -0,0 +1,66 @@
+import React from 'react';
+import { useRouter } from 'next/router';
+import '../styles/globals.css';
+
+const routes = [
+  '/',
+  '/ControlledUncontrolled',
+  '/CustomConnectionLine',
+  '/CustomNode',
+  '/DefaultNodes',
+  '/DragHandle',
+  '/DragNDrop',
+  '/EdgeTypes',
+  '/Edges',
+  '/Empty',
+  '/FloatingEdges',
+  '/Hidden',
+  '/Interaction',
+  '/Layouting',
+  '/MultiFlows',
+  '/NestedNodes',
+  '/NodeTypeChange',
+  '/NodeTypesObjectChange',
+  '/Overview',
+  '/Provider',
+  '/SaveRestore',
+  '/Stress',
+  '/Subflow',
+  '/Switch',
+  '/TouchDevice',
+  '/Undirectional',
+  '/UpdatableEdge',
+  '/UpdateNode',
+  '/UseKeyPress',
+  '/UseReactFlow',
+  '/UseUpdateNodeInternals',
+  '/Validation',
+];
+
+function MyApp({ Component, pageProps }) {
+  const router = useRouter();
+
+  const onRouteChange = (evt) => {
+    router.push(evt.target.value);
+  };
+
+  return (
+    <>
+      <header>
+        <a className='logo' href='https://github.com/wbkd/react-flow'>
+          React Flow Dev
+        </a>
+        <select defaultValue={router.pathname} onChange={onRouteChange}>
+          {routes.map((route) => (
+            <option value={route} key={route}>
+              {route === '/' ? 'Basic' : route.substring(1, route.length)}
+            </option>
+          ))}
+        </select>
+      </header>
+      <Component {...pageProps} />
+    </>
+  );
+}
+
+export default MyApp;