initial release

The core CMSIS-DAP functionality is the same as what pico-debug has been using for years, but TinyUSB was updated to 0.18.0 to provide RP2350 support and RP2350-specific changes were made to the code. As before with pico-debug, initialization code is executed and thrown away (to reduce the memory footprint), but now this 'preboot' code is in a formally separate project.