Improve spatial audio capabilities (high channel count, surround, etc.)

[padenot]

For now, the support is fairly minimal and a bit inconsistent.

[aboba]

Today, immersive/spatial capabilities are typically provided via WebAudio and container standards, such as:

• RFC 8486
• IAMF

So I'm curious what you think needs to be added in WebCodecs.

[padenot]

Those two came to mind yes, but I was also thinking of making it clear how layouts are supported, in relation to how the Web Audio API does things, and how to handle (e.g.) Opus mapping family 255.