Improve typings

Created by: aj-r

Fixes #2317 (closed).

• Never use the Object type. We should use object instead, or better yet, define a proper interface for those objects.
• Never use the Function type. Instead, define a more specific signature for that function.
• Changed several instances of type: string to more specific types (e.g. type: 'video' | 'audio'). This way typescript can catch typos, e.g. mediaPlayer.getTracksFor('vidoe') will cause a compile error.
• Added special overloads for on(). This way listeners automatically get the right event type. e.g.

  mediaPlayer.on(MediaPlayer.events.TEXT_TRACKS_ADDED, e => {
      // 'e' implicitly has a type of TextTracksAddedEvent.
      // So the properties e.type, e.enabled, e.index, and e.tracks are available,
      // but any other property will raise a typescript compile error.
      this.tracks = e.tracks;
  });

• I didn't add many JSDoc comments for now to make the diff easier to read. I'll probably add them in a separate PR.