Configurable minimum demand for trades

Addresses MC-163962 where villager demand decreases indefinitely. Paper adds a patch to fix this by preventing demand from going below zero. This patch adds a config option to allow the minimum demand to instead be configurable.

purpur-server/minecraft-patches/sources/net/minecraft/world/entity/npc/Villager.java.patch

@@ -22,6 +22,15 @@
                  this.startTrading(player);
              }
  
+@@ -503,7 +_,7 @@
+ 
+     private void updateDemand() {
+         for (MerchantOffer merchantOffer : this.getOffers()) {
+-            merchantOffer.updateDemand();
++            merchantOffer.updateDemand(this.level().purpurConfig.villagerMinimumDemand); // Purpur - Configurable minimum demand for trades
+         }
+     }
+ 
 @@ -707,7 +_,7 @@
  
      @Override