Support encapsulation of WebVTT in ISO-BMFF

[AtticusFetch]

Support encapsulation of WebVTT in ISO-BMFF
Spec: ISO/IEC14496-30

https://github.com/Dash-Industry-Forum/dash.js/blob/development/src/streaming/text/TextSourceBuffer.js#L342

Test Stream: https://storage.googleapis.com/shaka-demo-assets/sintel-mp4-wvtt/dash.mpd

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[welcome]

👋 Thanks for opening your first issue here! 👋

If you're reporting a 🐞 bug, please make sure you include steps to reproduce it. We get a lot of issues on this repo, so please be patient and we will get back to you as soon as we can.
To help make it easier for us to investigate your issue, please follow the contributing guidelines.