Fix support for arrow function doc extraction

[chrisvxd]

JS Docs aren't extracted when using exporting default arrow functions

/**
 * This is an example description for an example function.
 *
 * @param foo - Example describing string `foo`.
 * @returns Description of return value.
 */
export default async (foo: string): Promise<string> => {
  return JSON.stringify({ foo })
}

Method description and params are missing from output. After some investigation, it seems ts-morph isn't picking up the docs, so getJsDocs() returns an empty array.