Allow enum generation in addition to union types

[awdyson]

It'd be really convenient to be able to iterate over things like this:

export type TimeZoneCode =
  | 'Africa/Abidjan'
  | 'Africa/Accra'
  | 'Africa/Addis_Ababa'
  | 'Africa/Algiers'
  | 'Africa/Asmara'
  | 'Africa/Bamako'
  | 'Africa/Bangui'
  | 'Africa/Banjul'
  | 'Africa/Bissau'
  // ...

[phryneas]

To be honest, without someone already having opened an issue on this (#1889 by the way), I would not have the slightest idea what you are talking about.