Make TNT a bit more fun.

But not too much.

TNT is a bit underwhelming at the moment. We can make it a bit
more interesting by ejecting not just one or two itemstacks,
but a bunch of them. This code splits up the drops into
separate itemstacks that are 2-5 items together, which
results in generally roughly 10 itemstacks being ejected.

Since now we have multiple ejecta, it makes sense to tune
the ejecta velocities a bit better to get the appearance of
an actual explosion better. The items will not all start
with the same vertical velocity, since that would look
like fireworks. Instead we give them all a different vertical
speed.