Behaviour for ordering items before packing (analog of BoxPacker's ItemSorter).
Summary
Callbacks
Return a negative int if a sorts before b, positive if after, 0 if equal.
Callbacks
@callback compare(a :: ExBoxPacker.Item.t(), b :: ExBoxPacker.Item.t()) :: integer()
Return a negative int if a sorts before b, positive if after, 0 if equal.