Feature/determine text track kind based on CHARACTERISTICS

[lpommers]

This PR will...

will set the text track kind to either captions or subtitles based on CHARACTERISTICS attribute in the MediaPlaylist for the track.

Why is this Pull Request needed?

If a text tracks has CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog,public.accessibility.describes-music-and-sound, the native video element in Safari will set the kind property of the textTrack to captions instead of subtitles.

Are there any points in the code the reviewer needs to double check?

1. I wasn't sure here if there was any MediaPlaylist information here to analyze to make the distinction between captions and subtitles when making a native text track.

   hls.js/src/controller/timeline-controller.ts

   Lines 225 to 245 in 79895a4

   	private createNativeTrack(trackName: string) {
   	if (this.captionsTracks[trackName]) {
   	return;
   	}
   	const { captionsProperties, captionsTracks, media } = this;
   	const { label, languageCode } = captionsProperties[trackName];
   	// Enable reuse of existing text track.
   	const existingTrack = this.getExistingTrack(trackName);
   	if (!existingTrack) {
   	const textTrack = this.createTextTrack('captions', label, languageCode);
   	if (textTrack) {
   	// Set a special property on the track so we know it's managed by Hls.js
   	textTrack[trackName] = true;
   	captionsTracks[trackName] = textTrack;
   	}
   	} else {
   	captionsTracks[trackName] = existingTrack;
   	clearCurrentCues(captionsTracks[trackName]);
   	sendAddTrackEvent(captionsTracks[trackName], media as HTMLMediaElement);
   	}
   	}

   .
2. The same goes for a non-native textTrack. I wasn't sure if I should be adding CHARACTERISTICS information to captionsProperties or not.

   hls.js/src/controller/timeline-controller.ts

   Lines 247 to 266 in 79895a4

   	private createNonNativeTrack(trackName: string) {
   	if (this.nonNativeCaptionsTracks[trackName]) {
   	return;
   	}
   	// Create a list of a single track for the provider to consume
   	const trackProperties: TrackProperties = this.captionsProperties[trackName];
   	if (!trackProperties) {
   	return;
   	}
   	const label = trackProperties.label as string;
   	const track = {
   	_id: trackName,
   	label,
   	kind: 'captions',
   	default: trackProperties.media ? !!trackProperties.media.default : false,
   	closedCaptions: trackProperties.media,
   	};
   	this.nonNativeCaptionsTracks[trackName] = track;
   	this.hls.trigger(Events.NON_NATIVE_TEXT_TRACKS_FOUND, { tracks: [track] });
   	}

Resolves issues:

Closes #2472

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md