This allows the board to disable the onboard speaker until explicitly
enabled in user code.
Testing performed on a CPB:
* Touching the AUDIO pin with a fingertip no longer generates noise/buzz
* Generating a waveform with `simpleio.tone` produces no sound by default
* When the board.SPEAKER_ENABLE is configured as a digital output and
set True, `simpleio.tone` does produce sound
Note that while guides should include information about SPEAKER_ENABLE, it's
possible that some users who omitted it could view this as a breaking change.
They can fix it by simply adding code similar to
speaker_enable = digitalio.DigitalInOut(board.SPEAKER_ENABLE)
speaker_enable.direction = digitalio.Direction.OUTPUT
speaker_enable.value = True
before using the internal speaker.