String literals defined in another file don't get translated to propTypes

[ericyliu]

if I define a string literal in a separate file from the react component, and use that string literal in the props of the react component, the resulting proptypes dont contain that key.

i.e.
heading.constants.ts

export type LEVEL = '1' | '2' | '3';

heading.ts

import { LEVEL } from './heading.constants.ts';

interface Props {
  level: LEVEL,
}

export const Header = (props: Props) => {
  <div>{props.level}</div>
}

dist/index.ts

...
Header.propTypes = {};