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/list.html

@@ -24,7 +24,7 @@ const Item = {
   template: `<div>{{ msg }} <button @click="$emit('rm')">x</button></div>`
 }
 
-const App = {
+Vue.createApp({
   components: {
     Item
   },
@@ -51,9 +51,7 @@ const App = {
       }
     }
   }
-}
-
-Vue.createApp().mount(App, '#app')
+}).mount('#app')
 </script>
 
 <style>