Reimplement battery voltage read and display for Badger 2040 W

[GMMan]

This measures VSYS ADC on Pi Pico W, which should be OK accuracy since the 3.3V reference is provided from the Pico W's buck regulator. Battery display is only enabled for Badger 2040 W because I didn't reimplement the battery measuring logic for Badger 2040.

Nothing special needs to be done to restore pin 29 back to output because the driver in Pico SDK automatically sets it on starting SPI comms.

Resolves #26