Add CI: PHP lint (7.4 and 8.x) and unit tests

CI currently runs only CodeQL (JavaScript) and the WordPress.org deploy. Add a workflow with a PHP 7.4 / 8.x matrix that runs php -l over includes/ and paystack-forms.php, plus bash tests/run.sh. The plugin declares Requires PHP: 7.4, so syntax from newer PHP versions would fatal on those sites.