USB Improvements

* 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

watch-library/shared/watch/watch.h

@@ -88,6 +88,10 @@ bool watch_is_usb_enabled(void);
   */
 void watch_reset_to_bootloader(void);
 
+/** @brief Call periodically from app main loop to service CDC RX/TX.
+  */
+void cdc_task(void);
+
 /** @brief Reads up to len bytes from the USB serial.
   * @param file ignored, you can pass in 0
   * @param ptr pointer to a buffer of at least len bytes
@@ -96,4 +100,4 @@ void watch_reset_to_bootloader(void);
   */
 int read(int file, char *ptr, int len);
 
-#endif /* WATCH_H_ */
+#endif /* WATCH_H_ */