types: improve typings for shallowRef

[pikax]

fix #453

const STATUS_INITIAL = 'initial'
const STATUS_READY = 'ready'
const STATUS_INVALIDATING = 'invalidating'

type Status = 'initial' | 'ready' | 'invalidating'

const status = shallowRef<Status>(STATUS_INITIAL)

if (status.value === STATUS_READY) {
    status.value = STATUS_INVALIDATING
}

Explanation shallowRef<Status> is returning Ref<'initial'> | Ref<'ready'> | Ref<'invalidating'> so when you do

if (status.value === STATUS_READY) {
  // status === Ref<'ready'> 
}

Same as vuejs/core#1780