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-dom/__tests__/directives/vShow.spec.ts

@@ -5,16 +5,15 @@ import {
   nextTick,
   VNode
 } from '@vue/runtime-core'
-import { createApp, vShow } from '@vue/runtime-dom'
+import { render, vShow } from '@vue/runtime-dom'
 
 const withVShow = (node: VNode, exp: any) =>
   withDirectives(node, [[vShow, exp]])
 
-let app: any, root: any
+let root: any
 
 beforeEach(() => {
-  app = createApp()
-  root = document.createElement('div') as any
+  root = document.createElement('div')
 })
 
 describe('runtime-dom: v-show directive', () => {
@@ -27,7 +26,7 @@ describe('runtime-dom: v-show directive', () => {
         return [withVShow(h('div'), this.value)]
       }
     })
-    app.mount(component, root)
+    render(h(component), root)
 
     const $div = root.querySelector('div')
 
@@ -43,7 +42,7 @@ describe('runtime-dom: v-show directive', () => {
         return [withVShow(h('div'), this.value)]
       }
     })
-    app.mount(component, root)
+    render(h(component), root)
 
     const $div = root.querySelector('div')
 
@@ -59,7 +58,7 @@ describe('runtime-dom: v-show directive', () => {
         return [withVShow(h('div'), this.value)]
       }
     })
-    app.mount(component, root)
+    render(h(component), root)
 
     const $div = root.querySelector('div')
     const data = root._vnode.component.data
@@ -110,7 +109,7 @@ describe('runtime-dom: v-show directive', () => {
         ]
       }
     })
-    app.mount(component, root)
+    render(h(component), root)
 
     const $div = root.querySelector('div')
     const data = root._vnode.component.data