Modernize API with Promises backwards compatibly.

Legacy navigator.getUserMedia takes callbacks.

http://htmlpreview.github.io/?https://raw.githubusercontent.com/jan-ivar/mediacapture-main/compromises/getusermedia.html

The title of the pull request is "Modernice API with Promises backwards compatibly". I don't understand how this is backwards compatible when the callbacks are removed.

Could you add text explaining that?