test(types): add prop validator test

[HerringtonDarkholme]

Due to the limitation of TS, prop validator must be fully annotated.

defineComponent will produce prop type unknown in setup method because of contextual typing of object literal.

defineCopmonent({
  props: {
    validated: {
      type: String,
      validator(val) {
        return val !== ''
      }
    }
  },
  setup(props) {  
    props.validated // error, props is unknown
  }
})

The fix now is to add explicit annotation to the validator function to suppress contextual typing.

This pull request adds an example test for this.
refer to microsoft/TypeScript#38623