Fix issue with wrong inferred type of observable.map in TS 5.9+

[mbest]

This fixes #4573

The issue is that since TypeScript 5.9, a particular pattern for calling observable.map caused a compiler error. Comparing with the definition of new Map, I saw that we could fix this by using function overloads instead of the union type.

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[mweststrate]

Thanks for putting this PR and others up! I'll try to review later this or next week, as I've been out of the loop for a bit, I'll need some time to catch up

[mweststrate]

Looks great, thanks!