Feature/villager trades (#122)

* Add modify and removing for villager trades

- Add mixin for villager trades
- Add methods to register modify and removing
- Implement base for VillagerMixin to provide additional villager data
- Basic Access & Mixin change
- Add AT and AW
- Add overriding for max offers a villager or the wanderer can have

* Add rare check for wandering trader

* Remove todo comment

* rename some methods

* Solve reviews for #122

Move non api stuff into TradeRegistryData
Rename fields in MerchantOfferAccess
Move trade stuff into internal package
Mark internal trade classes as ApiStatus.Internal

* Minor refactors (discussed on Discord)

* Add doc for AbstractVillagerMixin

* Reformat code

* Update gradle.properties

Co-authored-by: Max <maxh2709@gmail.com>

forge/src/main/resources/META-INF/accesstransformer.cfg

@@ -36,4 +36,10 @@ public net.minecraft.world.storage.FolderName <init>(Ljava/lang/String;)V
 public-f net.minecraft.item.AxeItem field_203176_a # STRIPABLES
 public-f net.minecraft.item.ShovelItem field_195955_e # FLATTENABLES
 public-f net.minecraft.item.HoeItem field_195973_b # TILLABLES
+public-f net.minecraft.item.MerchantOffer field_222223_a # baseCostA
+public-f net.minecraft.item.MerchantOffer field_222224_b # costB
+public-f net.minecraft.item.MerchantOffer field_222225_c # result
+public-f net.minecraft.item.MerchantOffer field_222227_e # maxUses
+public net.minecraft.item.MerchantOffer field_222231_i # priceMultiplier
+public net.minecraft.item.MerchantOffer field_222232_j # xp
 public net.minecraft.item.ItemModelsProperties func_239420_a_(Lnet/minecraft/util/ResourceLocation;Lnet/minecraft/item/IItemPropertyGetter;)Lnet/minecraft/item/IItemPropertyGetter; # registerGeneric