Optique 0.5.0: Enhanced error handling and message customization

Optique 0.5.0 is now available, bringing enhancements to error handling, help text generation, and overall developer experience while maintaining full backward compatibility. The update introduces better code organization by refactoring the large `@optique/core/parser` module into three focused modules: `@optique/core/primitives`, `@optique/core/modifiers`, and `@optique/core/constructs`. Error handling is improved with automatic conversion of default value callback errors into parser-level errors, and the new `WithDefaultError` class allows for structured error messages. Customization of default values in help text is now possible via an optional third parameter to `withDefault()`, enabling descriptive text instead of actual values. Additionally, the release provides comprehensive error message customization across all parser types and combinators, allowing context-specific feedback. These improvements aim to make Optique more user-friendly, especially for building CLI applications that require clear error messages and helpful documentation, making this release a significant step forward for developers using Optique.