Add Laravel 13 and PHP 8.4 support

[iRajul]

Summary

• Allow illuminate/process ^13.0 so Pyman can install alongside Laravel 13 components.
• Add PHP 8.4 CI coverage for the current stable dependency lane on Ubuntu and macOS.
• Avoid a PHP warning when the configured Python environment path already exists as a file.

Validation

• php84 /opt/homebrew/bin/composer validate --strict
• php84 /opt/homebrew/bin/composer update --prefer-stable --no-interaction --prefer-dist resolved illuminate/process v13.7.0
• php84 ./vendor/bin/pint --test
• php84 ./vendor/bin/phpstan analyse --ansi
• php84 ./vendor/bin/pest --colors=always --display-warnings

I also checked the PHP 8.4 prefer-lowest lane locally. It passes, but older lowest transitive packages emit PHP 8.4 deprecations, so this keeps lowest-dependency coverage on PHP 8.3 and adds PHP 8.4 coverage to the stable dependency lane.

[kauffinger]

Thanks, good work!