Rename parameter lastSlotId to slotCount

[viciscat]

Changed lastSlotId to slotCount in the createAndAdd method of SlotRanges

The mapping made sense up until the enderchest where the method looked like this

createAndAdd(list, "enderchest.", 200, 27);

with 200 in firstSlotId and 27 in lastSlotId, which doesn't really make sense (unless you count integer overflows but I don't think that's the intended thing).
The code does a fori loop from 0 to lastSlotId and does firstSlotId + i. So I think slotCount is a pretty good name, size could work too.