Add a BATTERY pin to board pins

[dhalbert]

BATTERY was already added to the hallowing, but BATTERY is not an alias for D9 on Feathers. Would be convenient so you don't have to remember which pin. Suggested by @jedgarpark and @caternuson.

[arofarn]

Note : it's already board.VOLTAGE_MONITOR, at least on Feather M4 Express.

[sajattack]

Some clarification needed here. Do we want BATTERY as another alias in addition to VOLTAGE_MONITOR and do you want it for all adafruit boards or just feathers?

[dhalbert]

The idea is that there's a pin with a name that matches the silkscreen labeling on the boards ("BAT" or BATTERY").