Proposal: Native RP2040 variant

[JellyTitan]

I've built a 2040 based variant, with the working title of "Sofle Pico". I took all the stuff i loved from the previous iterations, and tried to make it easier to build for people new to the hobby. I'm preparing to send out for the 4th, and hopefully final, prototype. The README is mostly complete, but will need to be reformatted before a PR. The build log is mostly notes on what changed between iterations. There is a small punch-list remaining, mostly QMK related, that can wait until the hardware is finalized.

@brianlow @DaneEvans @josefadamcik Would you be so kind as to take a gander & let me know if there's features or design choices that could be improved?

One decision i'm struggling with - should the LEDs be omitted in favor of combo MX/choc footprints like the Sofle V1? The drag soldering required for the level shifter & and the non-standard SK6803 makes the build more difficult.