Add panning support in tracks

[jonikorpi]

Currently it’s possible to pan new instruments however you like, inside the connectInstrument function. But it’d be nice to be able to pan notes from tracks: [[0, { pan: -0.5 }], [0, { pan: 0.5 }]]. Adding support for that using StereoPannerNode would be easy and (apparently) performant.

However, some use cases will require using the much more expensive and complicated 3D panning nodes.

Do I add support for both, just one, or neither?

[jonikorpi]

A custom playNote function could deal with panning data extracted from the tracks, leaving it a userland problem.