fixed missed command registration for extension, 1.3.0, added optional version.

src/main/java/carpet/CarpetExtension.java

@@ -47,4 +47,6 @@ public interface CarpetExtension
     default void onPlayerLoggedOut(ServerPlayerEntity player) {}
 
     default void onServerClosed(MinecraftServer server) {}
+
+    default String version() {return null;}
 }