Feat: implement convolution channel configuration

[b-ma]

This implements the multichannel stuff for the ConvolverNode
cf. https://webaudio.github.io/web-audio-api/#Convolution-channel-configurations

There is still a todo regarding input number of channel changes (i.e. from stereo to mono), I guess we need to add a bit of bookkeeping to properly handle the tail of the channel that disappeared, but not completely sure how to do that yet.

todos

• input channel number change (delegate to another PR?)
• wpt check

[b-ma]

WPT results:

before

- # pass: 260
- # fail: 68
- # type error issues: 0

after

- # pass: 322
- # fail: 6
- # type error issues: 0

Note that this also has an impact on examples/benchmarks.rs since the IR is 2 channels, but results are still ok:

id  | name                 | duration (ms) | Speedup vs. realtime | buffer.duration (s)
13  | Convolution reverb   |       224.155 |                66.92 |             15

[orottier]

Alright, that looks nice.
I took me a while to understand the spec regarding multi channel, but as I see it know the number of input channels and the number of convolver channels determine the output channel count. And you create a second (identical) convolver in case the IR buffer has only one channel to handle stereo inputs.

[orottier]

Relates to #542