Pico: SPI

[mkellner]

I've added initial SPI support to the Pico. There is also a new target: pico/ili9341 to support it.
And a wiring guide for the display at: https://github.com/Moddable-OpenSource/moddable/blob/pico/documentation/displays/wiring-guide-generic-2.4-spi-pico.md.

The initial implementation is using SPI1 on the following pins:

MISO: 8
MOSI: 11
SCK: 10

The ili9341 is connected with the additional pins:

CS: 9
DC: 7
RST: 12

As the underlying Pico SPI APIs are blocking only at this point, so is the Moddable implementation.

[wilberforce]

@mkellner
Nice work