[cpplings] Add modern C++ (14/17/20) feature exercises

[XueSongTap]

Enhancement This sub-issue will add exercises covering modern C++ features to help users learn the syntax and libraries introduced in C++14, C++17, and C++20.

The exercises will include:

• C++14: Generic lambdas, auto return type deduction, variable templates
• C++17: Structured bindings, std::optional, std::variant, if-with-initializer
• C++20: Concepts, ranges, designated initializers

Understanding these modern C++ features will help with writing more concise and maintainable code in the MiniOB project.

Parent issue: #508 (add more cpplings case)