Elytra support (#761)

* Add elytra and firework related features

* VarInt8 never existed and 1.13 was singular

data/pc/common/features.json

@@ -12,6 +12,36 @@
       "latest"
     ]
   },
+  {
+    "name": "hasElytraFlying",
+    "description": "the elytra exists and players can fly with it",
+    "versions": ["1.9", "latest"]
+  },
+  {
+    "name": "fireworkMetadataVarInt7",
+    "description": "firework attached entity metadata is at index 7 and is varint",
+    "versions": ["1.11", "1.13.2"]
+  },
+  {
+    "name": "fireworkMetadataOptVarInt8",
+    "description": "firework attached entity metadata is at index 8 and is optvarint",
+    "versions": ["1.14", "1.16.5"]
+  },
+  {
+    "name": "fireworkMetadataOptVarInt9",
+    "description": "firework attached entity metadata is at index 9 and is optvarint",
+    "versions": ["1.17", "latest"]
+  },
+  {
+    "name": "fireworkNamePlural",
+    "description": "the firework entity used for elytra boosting is named fireworks_rocket",
+    "versions": ["1.11", "1.12.2"]
+  },
+  {
+    "name": "fireworkNameSingular",
+    "description": "the firework entity used for elytra boosting is named firework_rocket",
+    "versions": ["1.13", "latest"]
+  },
   {
     "name": "blockMetadata",
     "description": "block metadata is encoded in a separate metadata",