types: add AsyncComponentFactory type

[kiyui]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Typescript

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

The current type definition for ComponentOptions does not have a definition for async component factories and as such causes errors when using the syntax with vue-class-component.

[FeliciousX]

👍 need this. quite annoying I can't use async component with vue-class-component .

@Component({
  components: {
    HelloComponent
  }
})

this works..
BUT

@Component({
  components: {
    'dynamic-component': () => ({
      component : import( 'from/some/index.vue' ),
      loading : LoadingComponent    
    })
  }
})

this currently throws a type error. Does this PR fix this? :D :D :D :D

[FeliciousX]

to make it work i had to do

@Component<any>({ // <-- this
  components: {
    'dynamic-component': () => ({
      component : import( 'from/some/index.vue' ),
      loading : LoadingComponent    
    })
  }
})

:/

[kiyui]

@FeliciousX yes, this should fix this.

[backbone87]

This overload isnt sufficient. At least createElement needs overloads too.

I think it would be better to extend the AsyncComponent type directly:

export type AsyncComponent<Data=DefaultData<never>, Methods=DefaultMethods<never>, Computed=DefaultComputed, Props=DefaultProps>
  = AsyncComponentPromise<Data, Methods, Computed, Props>
  | AsyncComponentFactory<Data, Methods, Computed, Props>

export type AsyncComponentPromise<Data=DefaultData<never>, Methods=DefaultMethods<never>, Computed=DefaultComputed, Props=DefaultProps> = (
  resolve: (component: Component<Data, Methods, Computed, Props>) => void,
  reject: (reason?: any) => void
) => Promise<Component | EsModuleComponent> | void;

export type AsyncComponentFactory<Data=DefaultData<never>, Methods=DefaultMethods<never>, Computed=DefaultComputed, Props=DefaultProps> = () => {
  component: AsyncComponentPromise<Data, Methods, Computed, Props>;
  loading?: Component | EsModuleComponent;
  error?: Component | EsModuleComponent;
  delay?: number;
  timeout?: number;
}