[Firmware] Implement nrf52840 usb serial logger as a defmt logger

[TheButlah]

defmt requires a global logger. The nrf52840 support defmt-rtt out of the box, but unlike the esp32c3, we don't have USB JTAG, so we would need to read the RTT via a hardware probe + JTAG or SWD. I would prefer to implement defmt::Logger using the nrf's usb serial, to make logging really easy and provide a good way to verify that the device is working.

There is some example code here that demonstrates how to set up the nrf52 for usb serial. We should take that code and adapt it for the firmware, and then implement a global logger using it.

[TheButlah]

Most likely we will actually use defmt_bbq as explained in #93 , just have an async usb task that processes the queue.

[TheButlah]

Implemented in #101