WebUSB: Missing characters in REPL

[microbit-mark]

micro:bit support #24390
micro:bit support #24489
micro:bit support #26301

When typing commands into the REPL, it often does not transmit the full attribute resulting in a an error

Another user reports the case that characters get added too eg microbit.accelerometer.get_x() produces
microbit.accelerometer.gget_x()

[microbit-carlos]

This is possibly related to #109.