Ventcritter gamerules probabilities should be checked

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Description

#180 makes changes to how the rule works, by using probabilities instead of min/max values. This is done by counting the number of valid locations and getting the spawn amount from that.

This means every map will have a different number of spawn locations, and as such different spawns. Currently values that just looked good where thrown in, but they might require fine tuning by someone who's actually willing to go through that pain