Simplify mod initializers and prevent extensions from mixing into Carpet just to register themselves (#1396)

src/main/resources/fabric.mod.json

@@ -20,10 +20,10 @@
   "environment": "*",
   "entrypoints": {
     "client": [
-      "carpet.CarpetServer::CLIENT_INITIALIZER"
+      "carpet.CarpetServer::onGameStarted"
     ],
     "server": [
-      "carpet.CarpetServer::SERVER_INITIALIZER"
+      "carpet.CarpetServer::onGameStarted"
     ]
   },
   "mixins": [