types: Remove Function type from PropType type

[pikax]

fix #291

Gets the correct type for the prop

Similar to #301

interface A {
  a: String
}

defineComponent({
  props: {
    a: Object as PropType<A>
  },
  setup(props){
    props.a 
  }
})

[antfu]

Wow, that's quick! Confirmed working.

[antfu]

I have double-checked the code and I think it's alright, let's ship it with v0.6.0!