Make it possible to detect if connected PC is running or shut down (Controllers with USB, 32u4)

[M-Reimer]

For some USB device projects it would be handy to know if the PC, my device is connected to, is still running or has been shut down. This has become a problem especially with "newer" motherboards, which now usually always have power on the USB power leads.

A common way to do this is to check if "SOF" events are still generated. Possible place to detect this would be:
https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/USBCore.cpp#L760
In another project (STM32 without Arduino libraries), I just used a timer interrupt counting up a variable. Then I reset this to zero every time a SOF event happens.
In my other code, I can now check the value of my variable. If it reaches a defined limit, the PC must have been shut down as the "resets to zero" no longer happen.

There may be simpler/better ways to do this in the Arduino core libraries and some API also has to be defined. Maybe you could consider adding such a feature. Would be very useful!

[M-Reimer]

I want to add, that accepting this pull request:
#6964
would fix this issue.

[sandeepmistry]

This has been added in arduino/ArduinoCore-avr#54.