feat: create AllUnionFields type

[RebeccaStevens]

fix #996

TODO: Create a deep version of the type.

[Emiyaaaaa]

Is there a better name for it?

// @sindresorhus

[som-sm]

@RebeccaStevens IMO, the following implementation is simpler than the one proposed in this PR:

export type AllUnionFields<Union> = [Union] extends [NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray]
	? Union
	: Simplify<
	SharedUnionFields<Union> &
	{
		readonly [P in ReadonlyKeysOfUnion<Union>]?: ValueOfUnion<Union, P>;
	} & {
		[P in Exclude<KeysOfUnion<Union>, ReadonlyKeysOfUnion<Union> | keyof Union>]?: ValueOfUnion<Union, P>;
	}
	>;

type ValueOfUnion<Union, Key> = Union extends unknown ? Key extends keyof Union ? Union[Key] : never : never;

type ReadonlyKeysOfUnion<T> = T extends unknown ? keyof {
	[P in keyof T as IsEqual<{[Q in P]: T[P]}, {readonly [Q in P]: T[P]}> extends true ? P : never]: never
} : never;

This passes all the existing test cases and also uses utilities like ValueOfUnion and ReadonlyKeysOfUnion, which can be exposed as standalone types.

WDYT?

[som-sm]

And this would probably also need changes similar to this.

[RebeccaStevens]

@som-sm I agree, that implementation is nicer. I pushed an update with that one.