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-core/__tests__/helpers/resolveAssets.spec.ts

@@ -12,7 +12,6 @@ import { mockWarn } from '@vue/shared'
 
 describe('resolveAssets', () => {
   test('should work', () => {
-    const app = createApp()
     const FooBar = () => null
     const BarBaz = { mounted: () => null }
 
@@ -49,8 +48,9 @@ describe('resolveAssets', () => {
       }
     }
 
+    const app = createApp(Root)
     const root = nodeOps.createElement('div')
-    app.mount(Root, root)
+    app.mount(root)
     expect(component1!).toBe(FooBar)
     expect(component2!).toBe(FooBar)
     expect(component3!).toBe(FooBar)
@@ -78,7 +78,6 @@ describe('resolveAssets', () => {
     })
 
     test('not exist', () => {
-      const app = createApp()
       const Root = {
         setup() {
           resolveComponent('foo')
@@ -87,14 +86,14 @@ describe('resolveAssets', () => {
         }
       }
 
+      const app = createApp(Root)
       const root = nodeOps.createElement('div')
-      app.mount(Root, root)
+      app.mount(root)
       expect('Failed to resolve component: foo').toHaveBeenWarned()
       expect('Failed to resolve directive: bar').toHaveBeenWarned()
     })
 
     test('resolve dynamic component', () => {
-      const app = createApp()
       const dynamicComponents = {
         foo: () => 'foo',
         bar: () => 'bar',
@@ -112,8 +111,10 @@ describe('resolveAssets', () => {
           }
         }
       }
+
+      const app = createApp(Root)
       const root = nodeOps.createElement('div')
-      app.mount(Root, root)
+      app.mount(root)
       expect(foo).toBe(dynamicComponents.foo)
       expect(bar).toBe(dynamicComponents.bar)
       expect(baz).toBe(dynamicComponents.baz)