fix: Allow use in jsdom environments without window.CSS

[jdufresne]

Description

Before accessing window.CSS, the code now checks whether it exists. If it is undefined, the feature-detection path returns false, indicating no support.

This ensures compatibility with jsdom, commonly used in testing frameworks like Jest. Since jsdom does not implement window.CSS, calling it previously triggered errors such as:

VIDEOJS: ERROR: TypeError: Cannot read properties of undefined (reading 'supports')

For more context, see the upstream issue: jsdom/jsdom#3991

Specific Changes proposed

Before accessing window.CSS, the code now checks whether it exists. If it is undefined, the feature-detection path returns false, indicating no support.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

[welcome]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.32%. Comparing base (d203eab) to head (a56934c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9137      +/-   ##
==========================================
+ Coverage   84.31%   84.32%   +0.01%     
==========================================
  Files         120      120              
  Lines        8154     8154              
  Branches     1964     1964              
==========================================
+ Hits         6875     6876       +1     
+ Misses       1279     1278       -1     

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