GREEN: implement command execution

lib/mars_rover.dart

@@ -41,4 +41,21 @@ class Rover {
         break;
     }
   }
+
+  void execute(String commands) {
+    for (var i = 0; i < commands.length; i++) {
+      final command = commands[i];
+      switch (command) {
+        case 'L':
+          turnLeft();
+          break;
+        case 'R':
+          turnRight();
+          break;
+        case 'M':
+          moveForward();
+          break;
+      }
+    }
+  }
 }