pc: Add feature flag for lpVec3 entity velocity (#1206)

Author: GenerelSchwerz

data/pc/common/features.json

@@ -906,6 +906,11 @@
     "description": "spawn position includes a dimension component",
     "versions": ["1.21.9", "latest"]
   },
+  {
+    "name": "entityVelocityIsLpVec3",
+    "description": "entity velocity packets use length-prefixed vectors in blocks per tick",
+    "versions": ["1.21.9", "latest"]
+  },
   {
     "name": "gameRuleUsesResourceLocation",
     "description": "gamerule names use resource location syntax",