Modernize the PHP test matrix

[vanderlee]

What changed

• Replace Travis CI with GitHub Actions across PHP 5.4, 5.6, 7.4, 8.0, 8.4, and 8.5.
• Use PHPUnit versions compatible with both legacy and current PHP runtimes.
• Use Composer 2.2 for PHP 5.4 and 5.6; Composer 1 can no longer resolve packages from Packagist.
• Add a namespaced PHPUnit compatibility alias for the existing tests.
• Fix the custom test autoloader to use platform-independent path separators.
• Use the maintained checkout action, normalize the Composer homepage, and expose composer test.
• Replace the README's Travis badge with the GitHub Actions badge.

Why

The existing Travis configuration included retired runtimes and HHVM, while the development dependency was pinned to PHPUnit 3.6. The bootstrap also built source paths with Windows separators, which is not portable to Linux CI.

Validation

The replacement GitHub Actions matrix passes on PHP 5.4, 5.6, 7.4, 8.0, 8.4, and 8.5.