cli: make create-plugin add extension to app if possible

packages/cli/templates/default-plugin/dev/index.tsx.hbs

@@ -1,11 +1,11 @@
 import React from 'react';
 import { createDevApp } from '@backstage/dev-utils';
-import { {{ pluginVar }}, ExamplePage } from '../src/plugin';
+import { {{ pluginVar }}, {{ extensionName }} } from '../src/plugin';
 
 createDevApp()
   .registerPlugin({{ pluginVar }})
   .addPage({
-    element: <ExamplePage />,
+    element: <{{ extensionName }} />,
     title: 'Root Page',
   })
   .render();

packages/cli/templates/default-plugin/src/index.ts.hbs

@@ -1 +1 @@
-export { {{ pluginVar }} } from './plugin';
+export { {{ pluginVar }}, {{ extensionName }} } from './plugin';

packages/cli/templates/default-plugin/src/plugin.ts.hbs

@@ -9,7 +9,7 @@ export const {{ pluginVar }} = createPlugin({
   },
 });
 
-export const ExamplePage = {{ pluginVar }}.provide(
+export const {{ extensionName }} = {{ pluginVar }}.provide(
   createRoutableExtension({
     component: () =>
       import('./components/ExampleComponent').then(m => m.ExampleComponent),