Flag to choose alt MicroPython with SoundEffect.

[microbit-matt-hillsdon]

We also need matching stubs so reorg to tie these closer together.

Demos:

• https://review-python-editor-next.microbit.org/audio-sound-effect/?flag=audioSoundEffect
• https://review-python-editor-next.microbit.org/audio-sound-effect/?flag=none (only necessary as review branches auto-enable flags)

Perhaps in the future we could expose a setting for this rather than the flag.

[github-actions[bot]]

Preview build will be at https://review-python-editor-next.microbit.org/audio-sound-effect/

[microbit-matt-hillsdon]

Glitches:

• Underscores in parameter names render badly and are very common in this new API. Probably a Pyright rabbit hole though. Pre-existing issue #435
• We don't cope with * in the parameter list (accidental undefined). We could omit it? I think there are probably other places in the API that should have it but don't. Needs new issue or fix here (probably easy).
  • I've elided them from the signature in API in 9a7b3df2705a8d5a30a303239fd867132af20c67. Worth some discussion but perhaps not high priority to settle.

[microbit-matt-hillsdon]

Not a draft but relying on @microbit-robert to test on a device.