feat(TU20/21): Implement la's TU21 patch file

This adds the following:

TU20:
1. Librarians now have a small chance to offer name tags for sale
2. Witches can now spawn at light level 7 or less in any biome or cave
3. Skeletons & Wither Skeletons now rarely spawn anywhere in the Nether

TU21:
1. Minecart entity limit increased

Minecraft.World/Pig.h

@@ -23,6 +23,7 @@ public:
 	virtual bool useNewAi();
 
 protected:
+	virtual bool removeWhenFarAway();
 	virtual void registerAttributes();
 	virtual void newServerAiStep();