fix: add check for WEBVTT file header

[mister-ben]

• Adds a check for the WEBVTT header, to avoid an edge case where a VTT could be determined to be a TS segment.
• Adds a test case
• Fixes False positive for ts container prevents caption parsing #44

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.80%. Comparing base (e67a1d1) to head (e193e2a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   87.78%   87.80%   +0.01%     
==========================================
  Files          18       18              
  Lines        1277     1279       +2     
  Branches      338      338              
==========================================
+ Hits         1121     1123       +2     
  Misses        156      156              

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