feat: implemented BoardSide.direction

lib/game/components/board_side.dart

@@ -19,4 +19,9 @@ extension BoardSideX on BoardSide {
 
   /// Whether this side is [BoardSide.right].
   bool get isRight => this == BoardSide.right;
+
+  /// Direction of the [BoardSide].
+  ///
+  /// Represents the line along the [BoardSide] moves.
+  int get direction => isLeft ? -1 : 1;
 }

test/game/components/board_side_test.dart

@@ -23,5 +23,12 @@ void main() {
       expect(side.isLeft, isFalse);
       expect(side.isRight, isTrue);
     });
+
+    test('direction is correct', () {
+      const side = BoardSide.left;
+      expect(side.direction, equals(-1));
+      const side2 = BoardSide.right;
+      expect(side2.direction, equals(1));
+    });
   });
 }