fix(watch): allow to pass {deep: true} on ref primitives

[pikax]

if you pass {deep:true} and you are watching a primitive ref it will always be undefined

    const count = ref(0)
    let dummy
    watch(
      count,
      (c, prevCount) => {
        dummy = [c, prevCount]
      },
      {
        deep: true
      }
    )