fix(types): allow use PropType with Function

[pikax]

Allows retrieve the correct function type

{
  e: Function,
  // return type
  ee: Function as PropType<() => string>,
  // arguments + object return
  ff: Function as PropType<(a: number, b: string) => { a: boolean }>,
  // required + object return
  eee: {
    type: Function as PropType<() => { a: string }>,
    required: true
  },
  // required + arguments + object return
  fff: {
    type: Function as PropType<(a: number, b: string) => { a: boolean }>,
    required: true
  }
}

[yyx990803]

Nice job!