R1:9c432545e4bf

Implement macros as audio sources

Summary:
Fixes T3887. Basically:

- Macros with audio get passed to the `audio-source` behavior.
- This keeps track of where they are relative to the viewport as the user scrolls.
- When the user scrolls a "once" macro into view, and it reaches roughly the middle of the screen, we play the sound.
- When the user scrolls near a "loop" macro, we start playing the sound at low volume and increase the volume as the user scrolls.

This feels pretty good on both…