Interface Playable

The surface mwg/audio needs from a playing sound - exactly what HTMLAudioElement already provides, named here so a test (or a game with its own audio backend) can supply a fake without a real Audio element existing, which nothing outside a browser can create.

A backend that synthesises its sound instead of decoding a file (see StreamPlayable) has to answer the questions HTMLAudioElement answers implicitly: currentTime reads and seeks the playhead and holds the duration once a track that does not loop has ended; playbackRate is honoured (by resampling) or ignored but never throws; onended fires once at the end of a track that does not loop and never for one that does; and play() may return a promise that settles when the audio is allowed to start.