[tutorial] How to use function pointer to extend ISR in Arduino

Hello

This tutorial will teach you how to extend the ISR routine using handlers as function pointers.

Read the tutorial here:

https://atadiat.com/en/micros/e-arduino-trick-share-interrupt-service-routines-between-libraries-application/

I suggest this way to be added to all Arduino ISRs (even the ones in files like wiring.c from Arduino core).

Comments are most welcome!