This brings the pins in line with the documented [pinouts]. The only
"special" pins:
- GP0, GP1 are mentioned as the default UART, so we init them as
such and give them the TX and RX alternative names.
- GP16 is connected to the onboard neopixel DIN line and we assign it
the NEOPIXEL name. (Power of the neopixel is connected to the 3.3V
rail, not a pin. See [schematic].)
- GP26, GP27, GP28, GP29 have unambiguous ADC designations in the
[pinouts], so we assign the `A` analogue names as the Pico board
definition does.
[pinouts]: https://www.waveshare.com/wiki/RP2040-Zero#Pinouts
[schematic]: https://www.waveshare.com/w/upload/4/4c/RP2040_Zero.pdf
