feat: parse WebVTT from fmp4 segments.

[adrums86]

Feature:

Add a module to parse WebVTT from fmp4 segments.

This module is based off of the ISO/IEC 14496-30 spec for carrying timed text data in ISOBMFF/FMP4 segments.

• Parses the timescale from a WebVTT init segment.
• Parses cue text, styles and timing data from WebVTT content segments.
• Exports cue objects for each parsed WebVTT cue inside the content segment with the following format:

{
  cueText: 'Example cue text',
  start: 0,
  end: 1,
  settings: 'color: yellow;'
}

• parseSegment returns an array of parsed cue objects for each WebVTT segment parsed which, contains all cues from the given segment.
• Ignores both empty and invalid WebVTT cue data.