No include guards in `Arduino_LED_Matrix.h`

[yuriko0505]

https://github.com/arduino/ArduinoCore-renesas/blob/main/libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

This header file does not contain include guards, so multiple includes will result in redefinition errors.

[yuriko0505]

(It was posted before I wrote the content, sorry...)

This header file does not contain include guards, so multiple includes will result in redefinition errors.
Also, currently, the library consists only of header files.
I believe that separating it into header and source files would enhance maintainability.

This is my first time contributing to this Arduino OSS.
Are there any procedures I need to follow to contribute, or is someone else planning to address this improvement?