Add basic speaker-like sound playing

[jmmv]

The missing piece to be able to write some retro-style games or demos is the ability to play some sounds. I'd probably start mimicking the PLAY statement from Locomotive BASIC to generate speaker tones, but may need to research this further. Obviously we could go all fancy and support sound files too, but that should come later.

[severak]

I can provide PLAY implementation for javascript. I will look into it and when I have usable demo I would hand over it to you to integrate it.

[jmmv]

@severak That'd be great! Just having a prototype in Javascript to model the built-in PLAY command from would be helpful, as I know zero about how to generate sound right now.