End users cannot make ATAXX drops

[yjf2002ghty]

Currently fairyground uses "[]" in the FEN to judge whether a variant has pockets & drops.
However on ATAXX, this is not the case. We do not have "[]" in the FEN but we can drop pieces on the 8 squares near a same color piece. Users cannot drop these pieces correctly and they can only move the piece, making it unplayable.
The solution for this is to check the game rule whether a variant contains ATAXX dropping type, which is related with this issue: #27