Fix types for ff_*_multi() copyout overloads

[reinhrst]

I ran into the issue that types.d.ts data is not correct for functions with copyout options. E.g. ff_read_multi() returns [number, Record<number, Packet[]>] in the default call, but [number, Record<number, Packet[]>]

I don't know if this PR fixes the issue in a direction you approve of; things become quite verbose, but I didn't see a quick and simple alternative.

[Yahweasel]

I didn't update the types here because I couldn't be bothered to write out exactly these monstrous union types. I don't mind if somebody else writes them for me tho X-D

[reinhrst]

I didn't update the types here because I couldn't be bothered to write out exactly these monstrous union types. I don't mind if somebody else writes them for me tho X-D

Fair enough :)