Field pests

Add some pests to fields to make harvesting a little more chaotic. On breaking any RB-controlled crops, a probability can be set (based on the type of crop) for it to spawn a field pest - a seriously weakened silverfish. If a player broke the block, it spawns targeting them. This pull sets the chance at 1/1000 by default.

This is intended to make bulk-farming harder to bot by introducing a tiny chance of spawning slow, weak, minimum health silverfish. A normal player can easily deal with these pests with a single punch, but a bot can only do so by breaching the rules and reacting to the pest.

Hoping the days of trivial bot farming can end...