fix(type): fix incorrect ref unwrap

[antfu]

resolve #257

Align with vue-next
https://github.com/vuejs/vue-next/blob/c6217b4d46cbd3d9e250f2f84de4846d3484c42d/packages/reactivity/src/ref.ts#L30-L37

I am not sure how these lines work in our current codebase

// with init value: a = ref({ a: ref(0) })
// typeof a will be Ref<{ a: number }>
export function ref<S, T = unknown, R = HasDefined<S> extends true ? S : RefValue<T>>(
  raw: T
): Ref<R>;

I kept it for now. But since it's not in vue-next, should we just remove it in order to have better type consistency with vue-next?

Edit: Removed and it's exactly matched to vue-next now.

[pikax]

Once #311 is merged we should be able keep the vue-next types in sync easily

[antfu]

@pikax This seems to not be covered by #311. I have made it exactly matched to vue-next now.