Add new decorator Ref

[CarterLi]

Inspired by https://angular.io/api/core/ViewChild.

Vue#$refs is generally typed as { [key: string]: Vue | Element | Vue[] | Element[] }, and using it in TypeScript often requires a type cast. The proposed decorator tries to simplify its usage.

[kaorun343]

Hi.

Thanks for your PR.
I'll merge this after fixing the point I commented.
Also, there's a conflict but I will fix this.

[CarterLi]

Rebased

[Glandos]

I know it's old, but if you don't need ref renaming, you can follow the guide frome https://class-component.vuejs.org/guide/refs-type-extension.html:

  $refs!: {
    input: HTMLInputElement
  }

It's usually shorter, but it depends on your taste and your use case.