[IMPLEMENT] Expose more captions callbacks and events

[Jmilham21]

What needs to be implemented
Expose:

• ...the native event for onCaptionsChanged / onCaptionsList (Android), and jwplayer(_:captionTrackChanged:) / jwplayer(_:updatedCaptionList:) (iOS) need to be exposed in JS.
• ...the getCurrentCaptions function (Android) / currentCaptionsTrack property (iOS).

How should it be implemented

• Follow convention and pass the native event to the JS with the values from the Native Event.
• Follow convention for adding a bridged callback to native SDKs in JS.

Acceptance Criteria

• Native events are bubbled to the JS
• The new events are defined in index.js and index.d.ts
• iOS and Android are both covered
• Add function for getCurrentCaptions in JS / Native
• Add demo of this in Example app OR in README.md
  • This is now optional. We will update our props and callback list soon

Additional context
If there are any other captions events or helper functions you see, feel free to implement.

[Jmilham21]

It'd be worth considering the onCaptionText event as well. This is helpful for those who want to render captions outside of the player.

[Jmilham21]

Finished with the work found here: #99

This will be in the next release to NPM (version TBD)