Minor improvements to typings

Created by: aj-r

There are a few issues in index.d.ts that go against Typescript's best practices:

• 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.
  • e.g. instead of listener: Function, we should do something like listener: (e: any) => void. Although hopefully we can replace any with a more specific event interface.
  • We might want to write different overloads for MediaPlayerClass.on for each different type of event object. For example:

    ...
    on(type: 'allTextTracksAdded', listener: (e: AllTextTracksAddedEvent) => void, scope?: object);
    on(type: 'textTrackAdded', listener: (e: TextTrackAddedEvent) => void, scope?: object);
    on(type: 'canPlay', listener: () => void, scope?: object);
    ...

• It would also be nice to add JSDoc comments into index.d.ts so they show up in IDEs like Visual Studio Code. Ideally we could automate this, but for now it might be easiest to copy/paste them?

I can make a PR for this, since I assume it's not a high priority for most people. I can fill out the feedback agreement, but how do I send it to the mailing list? I'm not too familiar with how google groups works.