Set default pins for peripherals per datasheet (earlephilhower#103)

Using the official Raspberry Pi Pico datasheet and the Adafruit Feather
RP2040 schematic, set the default pins for peripherals to match.

Fixes earlephilhower#92

Remove api from include path, add redirects

Can't have cores/rp2040/api in the include path because the Arduino API
dir has a "String.h" file.  On Windows, because it is case-insensitive
normally, this overrides the POSIX "string.h" header leading to bad
stuff.

Add manual redirect includes for commonly accessed headers in the
cores/rp2040 path instead.

Fix JSON, Arduino 2.0 beta

Missing Size field was causing the Arduino 2.0 beta to reject the JSON.

Fix Picoprobe USB PID

Thanks to @me-no-dev for pointing out that the Picoprobe does UART
passthrough!

Fix Picoprobe USB PID

Thanks to @me-no-dev for pointing out that the Picoprobe does UART
passthrough!

Update README.md