Update library to PHP 8.3 with modern features

- Upgrade PHP requirement from 7.4 to 8.3
- Refactor Tracker class with constructor property promotion and readonly modifier
- Improve error handling with proper cURL resource checks and HTTP status validation
- Add timeouts for external API calls to prevent hanging requests
- Use JSON_THROW_ON_ERROR for safer JSON operations
- Enhance type declarations in Exception class
- Add development dependencies (PHPUnit, PHPStan, PHP CodeSniffer)
- Update README with requirements and modern usage examples
- Switch to HTTPS for geoip API calls for security