default `channelCountMode` for `AudioDestinationNode`
default `channelCountMode` for `AudioDestinationNode`

Without this, audioports would throw every time, preventing even the
manual testing from succeeding.

Kosar79/node-web-audio-api,corentingurtner/node-web-audio-api,sebpiq/node-web-audio-api,Kosar79/node-web-audio-api,sebpiq/node-web-audio-api