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 (1.21.1): NeoForge 21.1.1+, Forge 52.0.0+, or Fabric Loader 0.15.11+ with Fabric API 0.102.0+.