feat: modernize package with Guzzle HTTP client, exceptions and Laravel auto-discovery

[Elivandro]

Summary

This PR modernizes the RetroAchievements PHP API client with better standards and Laravel integration.

Changes

Core

• Replace native PHP HTTP calls with Guzzle HTTP client
• Return object (stdClass) instead of array for all API responses, allowing dot notation access ($profile->User)
• Add RetroAchievementsTrait to handle all HTTP logic separately from the service

Error Handling

• Add custom exception hierarchy (RetroAchievementsException, ApiException, InvalidApiKeyException, InvalidDateException)
• Prevent API key exposure in error messages
• Handle 401, 403, 4xx and 5xx responses with meaningful messages

Laravel Integration

• Add RetroAchievementsProvider with auto-discovery support
• Add RetroAchievements Facade
• Add publishable config file

Quality

• Add PHPStan level 8 static analysis
• Add Laravel Pint for code style (PSR-12)
• Add PHPUnit tests with Guzzle MockHandler (no real HTTP calls)

Documentation

• Update README with installation, usage and API reference in English
• Add public/index.example.php with standalone and Laravel usage examples

Breaking Changes

• API responses are now object instead of array — access with -> instead of []
• Constructor no longer accepts parameters — use setApiKey() instead

Testing

composer check

Related

Closes #