add PickEither

[uladkasach]

to start with, allow two options:

type PickOnly<T, U> = {
  [P in keyof T]: T[P];
} & {
  [P in keyof U]?: never;
};

type PickEither<T, U> = PickOnly<T, U> | PickOnly<U, T>;

https://stackoverflow.com/questions/37688318/typescript-interface-possible-to-make-one-or-the-other-properties-required

---

ideally, allow 2+ options

[uladkasach]

actually, that implementation above does not allow typescript to guarantee that pairs of types are defined together

so, will need a better impelmentation than that to star even