Add function to flip board.

lib/chess_bloc/chess_bloc.dart

@@ -49,7 +49,7 @@ class ChessBloc extends Bloc<ChessEvent, ChessBoardState> {
 }
 
 class ChessBoardState {
-  final bool flipped;
+  late bool flipped;
   final ChessColor turnColor;
   final Map<ChessCoordinate, ChessPiece> position;
 
@@ -111,4 +111,8 @@ class ChessBoardState {
 
     return ChessBoardState._(flipped, turnColor, position);
   }
+
+  void flipBoard() {
+    flipped = !flipped;
+  }
 }