v1.1: USB serial console and fixes

• Enables USB serial terminal in parallel with HID keyboard support
• Fixes "send" action not working correctly in literal mode
• Runs USB operations with an interrupt so rendering doesn't cause input lag

Initial build (fonts and uf2)

Here's a pre-built .uf2 firmware image for the Pi Pico, and a copy of the Unicode 14 font bundle I built and tested the device with (Unicode 15 wasn't released until halfway through the initial build).

I'm working on a Unicode 15 bundle, but haven't verified it on the device yet.