Improve documentation of IterableElement type & add tests for using with Set

[xeho91]

Added a new type SetKeys which is a helper to quickly infer the type of keys from a strongly typed Set or ReadonlySet.

[sindresorhus]

I think you can already use the IterableElement type for this.

[xeho91]

I think you can already use the IterableElement type for this.

Oh! I feel dumb missing this one. You're right, IterableElement gives same result.

Least I can do is update examples of IterableElement and mention keywords of the common Iterable instances (Set, Map) in the readme.md next to this type.

Especially this mention:

type-fest/source/iterable-element.d.ts

Line 36 in e75dbe1

Many types in JavaScript/TypeScript are iterables. This type works on all types that implement those interfaces. For example, `Array`, `Set`, `Map`, `stream.Readable`, etc.

Because I think is too hidden, at least for people like me. Hopefully not too many 😅

[sindresorhus]

👍

And also here: https://github.com/sindresorhus/type-fest?tab=readme-ov-file#alternative-type-names Should probably add SetValues there too, as that's a more correct term for it.