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/runtime-test/src/index.ts

@@ -2,7 +2,7 @@ import {
   createRenderer,
   VNode,
   RootRenderFunction,
-  App
+  CreateAppFunction
 } from '@vue/runtime-core'
 import { nodeOps, TestNode, TestElement } from './nodeOps'
 import { patchProp } from './patchProp'
@@ -14,7 +14,7 @@ const { render: baseRender, createApp: baseCreateApp } = createRenderer({
 })
 
 export const render = baseRender as RootRenderFunction<TestNode, TestElement>
-export const createApp = baseCreateApp as () => App<TestElement>
+export const createApp = baseCreateApp as CreateAppFunction<TestElement>
 
 // convenience for one-off render validations
 export function renderToString(vnode: VNode) {