Advice wanted.
So: one can subclass ShearItem and make an adequate custom shear item that will shear sheep just fine. The problem is, it won't cut leaf blocks like minecraft:shears will. I did a bit of spelunking in the code and loot tables, and found that leaves being obtainable by breaking with shears is specified in the loot-table for each leaf block, by tool name (e.g. "minecraft:shears"). Because of this questionable design decision, the only way I can see to make custom shears