LibChess: Add the ucinewgame command

This command is sent from the GUI to tell the engine that the next
position will be from a different game.

Userland/Libraries/LibChess/UCIEndpoint.h

@@ -31,6 +31,7 @@ public:
     virtual void handle_bestmove(BestMoveCommand const&) { }
     virtual void handle_info(InfoCommand const&) { }
     virtual void handle_quit() { }
+    virtual void handle_ucinewgame() { }
     virtual void handle_unexpected_eof() { }
 
     void send_command(Command const&);