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/classic/todomvc.html

@@ -82,14 +82,14 @@ const filters = {
   }
 }
 
-const App = {
+Vue.createApp({
   // app initial state
-  data: {
+  data: () => ({
     todos: todoStorage.fetch(),
     newTodo: '',
     editedTodo: null,
     visibility: 'all'
-  },
+  }),
 
   // watch todos change for localStorage persistence
   watch: {
@@ -192,7 +192,5 @@ const App = {
       }
     }
   }
-}
-
-Vue.createApp().mount(App, '#app')
+}).mount('#app')
 </script>