add custom GameEvent

src/main/java/com/example/tictactoe_game/GameEvent.java

+package com.example.tictactoe_game;
+
+import javafx.event.Event;
+import javafx.event.EventType;
+
+public class GameEvent extends Event {
+    public static final EventType<GameEvent> VICTORY_EVENT = new EventType<GameEvent>("VICTORY_EVENT");
+    public static final EventType<GameEvent> DRAW_EVENT = new EventType<GameEvent>("DRAW_EVENT");
+
+    public GameEvent(EventType<? extends Event> eventType) {
+        super(eventType);
+    }
+}