Villager Trade Swap

First release.

• A small swap button next to every trade in the trading screen replaces that one trade with a new one, without touching the others.
• The new trade comes from the same villager level as the old one, using the vanilla trade pool.
• Costs 1 emerald per swap by default (configurable on the server, free in Creative).
• A toggle above the trade list hides the buttons when you don't need them.

Requirements (1.20.1): Forge 47.1.0+ (also runs on NeoForge 1.20.1), or Fabric Loader 0.14.21+ with Fabric API 0.83.0+.