Improve typings

[aj-r]

Fixes #2317.

• 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.

[aj-r]

Technically Debug is an interface, not a class. This is because Debug objects are created with object literals ({}), not with a constructor (new Debug()).

[aj-r]

I'm pretty sure id should be a string, not a number. Can someone confirm?

[epiclabsDASH]

Good work @aj-r. Regarding typings, there is still space for improvement (some properties still declared as any, full lack of types for metrics, etc) but this is going in the good direction.

Thanks for the change.