Add support for audio/video attributes

- adds `currentTime` property for controlling position in a video or audio element
- adds `volume` property for controlling volume in a video or audio element
- adds `paused` mutation method for controlling play/pause state in a video or audio element
- adds `playbackRate` property for controlling playback speed in a video or audio element
- adds `srcObject` property for setting a WebRTC stream on a video element
- adds `kind` property/attribute for track element
- adds `srcLang` property/attribute for track element

Related: #1128 #1474 #2090
As I assume this is a supported attribute (or else it should be removed), not using `DOMMutationMethods` at all and using the attribute (as implemented by the standard) instead seems favorable.