Migrate CI to GitHub Actions

[vanderlee]

What changed

• Replace Travis CI with a GitHub Actions PHP matrix.
• Test PHP 8.1 and 8.4, which satisfy the current PHPUnit 10 development dependency.
• Validate Composer metadata, install dependencies, and run the existing PHPUnit suite.
• Restrict workflow permissions to read-only repository contents.

Why

The committed Travis configuration targets PHP 7.1/7.2 and HHVM, while the current package now requires PHP 8 and PHPUnit 10. The new workflow aligns CI with the repository's actual dependency constraints.

Validation

The draft workflow will run both PHP jobs on this pull request.