Play/pause now properly return Promises

[ffxsam]

Short description of changes:

This should fix errors such as "Uncaught (in promise) DOMException: The play() request was interrupted by a new load request", as developers can now use the returned Promise to wait for subsequent media actions.

Breaking in the external API:

N/A - though the documentation should be updated to let developers know that play, pause, and playPause now return a Promise as per the HTML5 audio spec (this PR should also work for WebAudio API as well, but please verify!)

Breaking changes in the internal API:

Not sure. Shouldn't be any.

Todos/Notes:

Might be a good idea to review WebAudio and HTMLAudio specs and check which methods return Promises, and return those from Wavesurfer's methods.

Related Issues and other PRs:

Issue #771 could be related.

[mspae]

Great idea!

The media element play method returns a promise, the pause method is currently not standardised and therefore we cannot rely on it returning a promise.

In general I think it is a good idea to return the play promise as you said. I think adjusting the JSdoc comments above the functions are still missing. (Maybe also with an @example block with an example of how to use the promise)

Regarding the implementation for WebAudio the specs should be useful: AudioContext.resume and AudioBufferSourceNode.start

[ffxsam]

That's fine if pause doesn't return a promise. In the code I've written, it should just return undefined.

I can update the JSDoc blocks when I get some spare time again.