app（ceateAppApi）

createAppAPI 的核心作用就是生成 app，里面包含一些常见的方法（use、component...）

export function createAppAPI(
  render: RootRenderFunction,
  hydrate?: RootHydrateFunction
) {
  return function createApp(rootComponent, rootProps = null) {
    const app: App = (context.app = {
      use(plugin: Plugin, ...options: any[]) {},
      mixin(mixin: ComponentOptions) {},
      component(name: string, component?: Component): any {},
      directive() {},
      mount(){},
      unmount() {},
      provide(key, value) {}
    })
    return app
  }
}