refactor: adjust createApp related API signatures

BREAKING CHANGE: `createApp` API has been adjusted.

  - `createApp()` now accepts the root component, and optionally a props
  object to pass to the root component.
  - `app.mount()` now accepts a single argument (the root container)
  - `app.unmount()` no longer requires arguments.

  New behavior looks like the following:

  ``` js
  const app = createApp(RootComponent)
  app.mount('#app')
  app.unmount()
  ```

packages/vue/examples/transition/modal.html

@@ -56,13 +56,12 @@ const Modal = {
 </div>
 
 <script>
-const App = {
+Vue.createApp({
   components: { Modal },
-  data: {
+  data: () => ({
     showModal: false
-  }
-}
-Vue.createApp().mount(App, '#app')
+  })
+}).mount('#app')
 </script>
 
 <style>