Support for the PHPUnit dialect of JUnit

Refactor PHPUnit support into separate phpunit-junit parser

Instead of modifying the Java JUnit parser, this creates a dedicated
PHPUnit parser that properly handles PHPUnit's nested testsuite elements.
This keeps the parsers cleanly separated and allows for future PHPUnit-
specific features.

Co-Authored-By: Matteo Beccati <matteo@beccati.com>
Co-Authored-By: Claude Code <noreply@anthropic.com>

action.yml

@@ -32,6 +32,7 @@ inputs:
         - java-junit
         - jest-junit
         - mocha-json
+        - phpunit-junit
         - python-xunit
         - rspec-json
         - swift-xunit