SharedUnionFields: Simplify implementation

[som-sm]

Simplifies the implementation of SharedUnionFields type, as mentioned here. ShareUnionFields is essentially just Pick with an extra conditional for non-recursive types.

Also, adds test cases to cover the non-recursive check, as mentioned here.

[som-sm]

@Emiyaaaaa, IMO we should skip non-recursive types even if they are mixed with recursive ones, as suggested here.

For eg., { a: 1 } | { a: 2 } | undefined currently evaluates to {} but I think should instead evaluate to { a: 1 | 2 } | undefined.

And, here's a possible implementation for the same:

type SharedUnionFields<Union> =
Exclude<Union, NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray> extends infer ProcessableMembers
	?
	| Exclude<Union, ProcessableMembers>
	| (IsNever<ProcessableMembers> extends true ? never : Simplify<Pick<ProcessableMembers, keyof ProcessableMembers>>)
	: never;

WDYT?

[Emiyaaaaa]

And, here's a possible implementation for the same:

type SharedUnionFields<Union> =
Exclude<Union, NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray> extends infer ProcessableMembers
	?
	| Exclude<Union, ProcessableMembers>
	| (IsNever<ProcessableMembers> extends true ? never : Simplify<Pick<ProcessableMembers, keyof ProcessableMembers>>)
	: never;

I think this is better

[som-sm]

@Emiyaaaaa I've updated the PR, please have a look.

I had to tweak the solution a bit from my earlier proposal to ensure it returns any for any.