* Introduce shell module for basic serial shell with argument parsing
* Introduce shell_cmd_list module for basic compile-time command
registration
* Harden USB handling to hang less and drop fewer inputs
- Service tud_task() with periodic TC0 timer interrupt
- Service cdc_task() with periodic TC1 timer interrupt
- Handle shell servicing in main app loop
- Add a circular buffering layer for reads/writes
* Change newline prints to also send carriage return
* Refactor filesystem commands for shell subsystem
* Introduce new shell commands:
- 'help' command
- 'flash' command to reset into bootloader
- 'stress' command to stress CDC writes
Testing:
* Shell validated on Sensor Watch Blue w/ Linux host
* Shell validated in emscripten emulator
* Tuned by spamming inputs during `stress` cmd until stack didn't crash
