fix(media-groups): set text track kind to captions based on HLS characteristics

[amtins]

Description

Previously, all HLS SUBTITLES media group renditions were assigned the
'subtitles' text track kind by default.

According to the W3C and WHATWG HTML standards, tracks that transcribe dialogue
or describe music and sounds are called captions. They're intended to help
people who are deaf or hard of hearing.

The HLS RFC 8216 standard allows CHARACTERISTICS to contain several
comma-separated values, such as public.easy-to-read. The kind is now
determined by checking each value separately.

Specifications references:

• HLS RFC 8216 Section 4.3.4.1 (ref Characteristics):
  https://datatracker.ietf.org/doc/html/rfc8216#section-4.3.4.1

  A SUBTITLES Rendition MAY include the following characteristics:
      "public.accessibility.transcribes-spoken-dialog",
      "public.accessibility.describes-music-and-sound", and
      "public.easy-to-read" (which indicates that the subtitles have
      been edited for ease of reading).
  

• HLS RFC 8216 Section 8.9:
  https://datatracker.ietf.org/doc/html/rfc8216#section-8.9

  CHARACTERISTICS Attribute Containing Multiple Characteristics
  
   Certain characteristics are valid in combination, as in:
  
   CHARACTERISTICS=
   "public.accessibility.transcribes-spoken-dialog,public.easy-to-read"
  

• WHATWG HTML / W3C HTML5 Track Kind 'captions':
  https://html.spec.whatwg.org/multipage/media.html#attr-track-kind-keyword-captions
  https://www.w3.org/TR/2011/WD-html5-20110113/video.html#the-track-element

  captions
  
      Closed captions provide transcription or translation of the dialog, sound effects, relevant musical cues, and other relevant audio information, such as the cue's source (e.g., character, environment). They are suitable for when sound is unavailable or not clearly audible (e.g., because it is muted, drowned-out by ambient noise, or because the user is deaf).
  

Specific Changes proposed

• add a private function named textTrackKind_ that will determine the correct kind
• add test cases

Requirements Checklist

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.03%. Comparing base (a9f9d7a) to head (af45ac6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1616      +/-   ##
==========================================
+ Coverage   84.00%   84.03%   +0.02%     
==========================================
  Files          44       44              
  Lines       11713    11720       +7     
  Branches     2625     2629       +4     
==========================================
+ Hits         9840     9849       +9     
+ Misses       1873     1871       -2     

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