Improve code readability by using concepts where appropriate - #556

Summary of Changes

Hello @liuzicheng1987, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a significant refactoring effort to modernize the codebase by adopting C++20 concepts. The primary goal is to enhance code readability and maintainability by replacing verbose std::enable_if template constraints with the more concise and expressive requires clauses. Additionally, it improves the modularity of the parsing system by creating a dedicated parser for enum types, streamlining their handling and making the codebase easier to navigate and extend.

Highlights

• Modernized Template Constraints: Replaced numerous instances of std::enable_if with C++20 requires clauses across various header files (e.g., Attribute.hpp, Binary.hpp, Field.hpp, Generic.hpp, Result.hpp, TaggedUnion.hpp, Validator.hpp, Variant.hpp, Skip.hpp), significantly improving the readability and conciseness of template metaprogramming.
• Simplified Type Trait Definitions: Updated type trait checks in flexbuf/Reader.hpp and internal/has_to_class_method_v.hpp to leverage C++20 concepts, making them more expressive and easier to understand than their std::enable_if counterparts.
• Improved Modularity for Enum Parsing: Extracted all enum-related parsing logic from Parser_default.hpp into a new, dedicated file Parser_enum.hpp. This change centralizes enum serialization, deserialization, and schema generation, enhancing the overall structure and maintainability of the parsing components.

Why not using std::convertible_to?