Add temporary backwards compatability for ServerHelper

src/main/java/emu/grasscutter/plugin/api/ServerHelper.java

@@ -15,7 +15,7 @@ import java.util.List;
 import java.util.stream.Stream;
 
 /** Hooks into the {@link GameServer} class, adding convenient ways to do certain things. */
-public final class ServerHelper {
+public class ServerHelper {
     private static ServerHelper instance;
     private final GameServer gameServer;
     private final HttpServer httpServer;