Added OnGameServerStop EventBus

There is a temporary warning message telling users to ignore the 'No subscribers registered' error. This can be removed once things actually subscribe to it

src/main/java/emu/grasscutter/server/game/GameServerStopEvent.java

@@ -0,0 +1,5 @@
+package emu.grasscutter.server.game;
+
+public class GameServerStopEvent {
+    // Placeholder class for now, probably will get used later
+}