In Arduino micro/leonardo Pin HWB is not exposed as digital pin

In the [Atmega32u4 Pin Mapping](https://www.arduino.cc/en/Hacking/PinMapping32u4) PE2 is documented as HWB so the expected behavior would be the user to be able to use it as a digital pin.
In the official boards (leonardo & micro) is tied to ground but is totally usable if you are using it with a atmega32u4 board.
The pull down on this pin is not required as the default fuses do not have the HWBE bit set, meaning that is completely ignored during bootup.
So make sense to Included in the pins_arduino.h a mapping for as arduino digital pin 31 and Alias name HWB.

Start in pins_arduino.h and compare the ATmega32u4 pin mapping with the Arduino documentation for PE2/HWB. Done means PE2 is exposed as digital pin 31 with the HWB alias for compatible boards, while existing Leonardo and Micro mappings remain correct.

Written by the indexing model from the issue text.