Add 2 new methods mappings and rename a few others. (#516)

* Change ItemStack#invalid to ItemStack#empty. Only ever used when checking if the stack is empty

* Rename BucketMilkItem to MilkBucketItem

* Change FireworksItem to FireworkItem (registry name is also singular, not plural)

* Rename EntityType.ILLAGER_BEAST to RAVAGER to match the class / actual entity name.

* Map 2 of FenceBlock's connection-related methods.

mappings/net/minecraft/block/FenceBlock.mapping

@@ -2,6 +2,8 @@ CLASS bkf net/minecraft/block/FenceBlock
 	FIELD i SHAPES [Lcof;
 	METHOD <init> (Lbig$c;)V
 		ARG 1 settings
+	METHOD a canConnect (Lbre;ZLex;)Z
+	METHOD f preventsConnection (Lbig;)Z
 	METHOD k (Lbre;Lbcq;Les;)Lcof;
 		ARG 1 state
 		ARG 2 view

mappings/net/minecraft/entity/EntityType.mapping

@@ -57,7 +57,7 @@ CLASS ahq net/minecraft/entity/EntityType
 	FIELD aR ZOMBIE_HORSE Lahq;
 	FIELD aS ZOMBIE_VILLAGER Lahq;
 	FIELD aT PHANTOM Lahq;
-	FIELD aU ILLAGER_BEAST Lahq;
+	FIELD aU RAVAGER Lahq;
 	FIELD aV LIGHTNING_BOLT Lahq;
 	FIELD aW PLAYER Lahq;
 	FIELD aX FISHING_BOBBER Lahq;

mappings/net/minecraft/item/BucketMilkItem.mapping

@@ -1 +0,0 @@
-CLASS ayd net/minecraft/item/BucketMilkItem

mappings/net/minecraft/item/FireworkItem.mapping

@@ -1,4 +1,4 @@
-CLASS axg net/minecraft/item/FireworksItem
+CLASS axg net/minecraft/item/FireworkItem
 	CLASS axg$a Type
 		FIELD f TYPES [Laxg$a;
 		FIELD g id I

mappings/net/minecraft/item/ItemStack.mapping

@@ -6,7 +6,7 @@ CLASS axx net/minecraft/item/ItemStack
 	FIELD e updateCooldown I
 	FIELD f item Laxs;
 	FIELD g tag Lhx;
-	FIELD h invalid Z
+	FIELD h empty Z
 	FIELD i holdingItemFrame Lapi;
 	FIELD j lastCheckedCanHarvestBlock Lbri;
 	FIELD k lastCheckedCanHarvestResult Z

mappings/net/minecraft/item/MilkBucketItem.mapping

@@ -0,0 +1 @@
+CLASS ayd net/minecraft/item/MilkBucketItem