Expose KeysOfUnion type

[henriqueinonhe]

Adds the AllKeys type, that is similiar to keyof but it distributes over unions, that is, it returns the keys of ALL members of a union type.

Although this is a first step to add the types in #132, it is also useful by itself.

Additionally, I only used this name (AllKeys) because it's what was suggested in the issue I mentioned above, but there are other possible names we might want to consider, like KeyOfDistributeUnions or DistributedKeyOf.

[sindresorhus]

You need to add it to the readme too.

[sindresorhus]

Some more naming alternatives:

• AllMemberKeys
• EveryKeyOf

[sindresorhus]

Hmm, we actually have a type for this already:

type-fest/source/internal.d.ts

Lines 40 to 47 in 47626cf

	/**
	Gets keys from a type. Similar to `keyof` but this one also works for union types.
	The reason a simple `keyof Union` does not work is because `keyof` always returns the accessible keys of a type. In the case of a union, that will only be the common keys.
	@link https://stackoverflow.com/a/49402091
	*/
	export type KeysOfUnion<T> = T extends T ? keyof T : never;

I think that name is the best.

[henriqueinonhe]

Interesting... Would it be okay to expose this internal type then?

[sindresorhus]

You can just replace the use of that type with yours (make sure you merge any docs/tests for it with yours).

[henriqueinonhe]

Merged docs, replaced uses.
There were no preexisting tests for this specific type, so I left the ones I added.