feat: add unref API to match API with vue3

[pikax]

Same implementation as vue-next

https://github.com/vuejs/vue-next/blob/a51b05267231fb1770d6fc727bc276475c21d27f/packages/reactivity/src/ref.ts#L69-L71

export function unref<T>(ref: T): T extends Ref<infer V> ? V : T {
  return isRef(ref) ? (ref.value as any) : ref
}

[pikax]

merged into #309

[casprwang]

can we make a release for this API? It's fairly useful.