[mediacapture-record] Provide an option to enumerate all supported media types

sole has just created a new issue for 
https://github.com/w3c/mediacapture-record:

== Provide an option to enumerate all supported media types ==
Right now the method `isTypeSupported` provides *some amount* of media
 support detection, but developers might not be able to come up with 
all the possible combinations of codec and rate (e.g. 
`aType=video/webm; codecs="vp9"` vs `aType=video/mp4; 
codecs="avc1.4d4015"; width=426; height=240`, etc

Could it be possible to provide a method that enumerates the available
 options, so that developers can provide that to the users?

This is an often requested feature when talking to developers about 
`MediaRecorder`

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-record/issues/59 using your GitHub
 account