led_matrix: add compatibily layer with UNO R4

Author: facchinm

libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

@@ -236,4 +236,7 @@ class Arduino_LED_Matrix
     bool _loop = false;
     bool _sequenceDone = false;
     voidFuncPtr _callBack;
-};
+};
+
+// For backwards compatibility, also define the class name without underscores
+typedef Arduino_LED_Matrix ArduinoLEDMatrix;