GREEN: implement move forward

lib/mars_rover.dart

@@ -24,4 +24,21 @@ class Rover {
     };
     direction = rightTurns[direction]!;
   }
+
+  void moveForward() {
+    switch (direction) {
+      case 'N':
+        y += 1;
+        break;
+      case 'E':
+        x += 1;
+        break;
+      case 'S':
+        y -= 1;
+        break;
+      case 'W':
+        x -= 1;
+        break;
+    }
+  }
 }