Note: this change will break non-deep `watch` on the `v-model` bound array since the array is no longer replaced. This can be considered part of the Array watch changes in v3 as detailed at https://v3.vuejs.org/guide/migration/watch.html
This is unfortunate but unavoidable since the issue that it fixes is more important: `v-model` should definitely work with a non-ref reactive array.
fix#2662
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()
```
