refactor: use URL API

[mister-ben]

Description

Replaces some now unnecessary fallbacks with the standard URL API.
parseUrl new returns an actual URL instead of a URL-like object.
Updates tests slightly: getAbsoluteURL() to match URL's behaviour with trailing slashes, and use a URL object in the cross origin tests.
Adds a test for getAbsoluteURL() with a relative URL

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
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.97%. Comparing base (cb76a24) to head (0679fdb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8716      +/-   ##
==========================================
- Coverage   83.00%   82.97%   -0.04%     
==========================================
  Files         120      120              
  Lines        7998     7978      -20     
  Branches     1927     1921       -6     
==========================================
- Hits         6639     6620      -19     
+ Misses       1359     1358       -1     

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