Added: Support for AUDIO SOURCE tags

[dajanvulaj]

These changes add support for AUDIO SOURCE elements while supporting the src attribute added directly in the AUDIO element if no SOURCE children are provided.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.44%. Comparing base (214e551) to head (97916b1).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
...es/fwfh_just_audio/lib/src/internal/tag_audio.dart	75.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
- Coverage   97.58%   97.44%   -0.14%     
==========================================
  Files          79       79              
  Lines        4922     4936      +14     
==========================================
+ Hits         4803     4810       +7     
- Misses        119      126       +7     

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