Spec better codec strings for wav ulaw / alaw

[chcunningham]

wav files are described with mime types like:
'audio/wav; codecs=1'

I believe codecs=1 signals 'Microsoft PCM Format', as described in this RFC: https://www.ietf.org/rfc/rfc2361.txt

Other notable digits include 6:ulaw and 7:alaw. Chrome recognizes 1 via canPlayType() and kin. Firefox recognizes more. Haven't checked the other UAs.

Such codecs strings aren't very readable, particularly if used in WebCodecs without the accompanying 'audio/wav' prefix. Strawman solution: revive the idea to have a registry of codecs strings. This will serve to document the nice familiar strings for other codecs and give us an option to set a new precedent for the wav strings (e.g. 'pcm', 'ulaw', 'alaw').