Fix: In-manifest VTT iOS MSE issue

[dzianis-dashkevich]

Description

We have a bug with default options setup for HLS in-manifest VTT in chrome iOS (and I assume other non-safari Webkit-based iOS browsers). The problem is that we do not load vtt.js, but we use VHS to handle playback. vtt-segment-loader implicitly depends on window.WebVTT.Parser. Since vtt.js is not loaded - window.WebVTT is undefined. It means that we are not able to parse received VTT cues.

Specific Changes proposed

• Do not override native for all iOS/iPadOS browsers by default.
• Add fallback logic to load vtt.js in vtt-segment-loader if we do not have it loaded. (eg: force overrideNative option case).
• Add exception guard with a specific error message in case we call parse but still do not have vtt.js loaded for any reason.
• Fix/Add tests to cover all listed changes.

Note Tested Chrome on iPadOS 16.2: default run: (native playback and apple's native text tracks are shown) set override Native to true (VHS playback and apple's native text tracks are shown) set override Native to true and set native text tracks to false (VHS playback and styled text tracks are shown)

Requirements Checklist

• [x] Feature implemented / Bug fixed
• [ ] If necessary, more likely in a feature request than a bug fix
  • [x] Unit Tests updated or fixed
  • [ ] Docs/guides updated
  • [ ] Example created (starter template on JSBin)
• [ ] Reviewed by Two Core Contributors

[codecov[bot]]

Codecov Report

Merging #1360 (5b877a9) into main (d7e8713) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1360      +/-   ##
==========================================
+ Coverage   86.01%   86.03%   +0.01%     
==========================================
  Files          40       40              
  Lines        9842     9855      +13     
  Branches     2294     2295       +1     
==========================================
+ Hits         8466     8479      +13     
  Misses       1376     1376              

Impacted Files	Coverage Δ
src/playlist-controller.js	95.18% <100.00%> (+0.06%)	:arrow_up:
src/videojs-http-streaming.js	90.61% <100.00%> (+0.13%)	:arrow_up:
src/vtt-segment-loader.js	80.79% <100.00%> (-0.33%)	:arrow_down:

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