Feature request: Add BOOTSEL to protocol

[usr97629238]

Since the RP2040 (when used as an oscilloscope) is usually inside some kind of an enclosure, it would be nice to extend the protocol with a switch to reboot it into BOOTSEL mode.

[pico-coder]

As part of the major rewrite I added bootsel support to the protocol.
If connect the device, open a serial terminal, and send a serial command of "bootsel" (without parens) the device will print "Entering Bootsel mode" and then enter bootsel mode. Any of the uf2s in the release directory that are the "UF2s for the major 2025 rewrite" should exhibit the behavior, but the pico_original*uf2 files will not.
Please try it out, and post results back here.