types(reactivity): creating type for return toRefs and export return type

[pikax]

expose return type for toRefs

Some Typescript users might have some trouble when using reactive and with toRefs

interface Foo {
  a: number,
  b: number
}

interface FooReturn {
  foo: ToRefs<Foo> // naming might be confusing, suggestions welcome
}

function useFoo(){
  const state = reactive({
    a: { a:1, b:2}
  })

  return {
     foo: toRefs(state.a)
  }
}