CustomStateSet missing Set methods

[zzzzBov]

CustomStateSet appears to be defined as:

interface CustomStateSet {
    forEach(callbackfn: (value: string, key: string, parent: CustomStateSet) => void, thisArg?: any): void;
}

but it's meant to have methods and properties similar to Set, which MDN confirms.

[saschanaz]

This is a config issue, see also #1724 (comment).

@github-actions close

[github-actions]

Closing because @saschanaz is one of the code-owners of this repository.