ChessEngine: Handle the UCI quit command

Author: https://github.com/tcl3
Commit: c2b9376409
Pull-request: https://github.com/SerenityOS/serenity/pull/18140
Issue: https://github.com/SerenityOS/serenity/issues/17427
Reviewed-by: https://github.com/LucasChollet
Reviewed-by: https://github.com/petelliott

Userland/Services/ChessEngine/ChessEngine.h

@@ -18,6 +18,9 @@ public:
     virtual void handle_uci() override;
     virtual void handle_position(Chess::UCI::PositionCommand const&) override;
     virtual void handle_go(Chess::UCI::GoCommand const&) override;
+    virtual void handle_quit() override;
+
+    Function<void(int)> on_quit;
 
 private:
     ChessEngine() = default;