Improve code by using modern C++ features

[fel1x-developer]

Half-Life SDK uses old C++ (probably C++98) C++20 can make code safer, easier to read, and faster. Using modern C++ features, halflife-updated will be easier to write code, add features, and debug.

Changes

• Use C++20.
• Replace #define macros with enum or constexpr. This makes fixing compile-time errors easier. Use enum for group of constants and constexpr for individual constants.
• Replace macro functions with inline functions.
• Replace typedef with using. using is added in C++.
• Replace typedef struct with struct. C++ structs work without typedef.
• Use mathematical constants added in C++20. (pi and pi_v)
• Replace qboolean with bool
• Replace unsigned char with std::uint8_t for byte.
• Use auto where it is possible.
• Replace printf with std::cout. But std::endl won't replace \n.
• Use decltype or vec_t to avoid float and double collision for vec_t.
• Use C++ headers.
• Use pragma once for header files.
• Replace array with std::array. C++ std::array is safer.
• Replace call by pointer with call by reference where it is possible.
• Replace C standard functions (malloc, free) with C++ functions (new, delete).
• Replace raw pointers to smart pointers (unique_ptr, shared_ptr)
• Use lambda functions where it is possible.
• Replace std::string with char* strings. ``std::string:iteratorcan be used. Replace related functions such asatof` with `stof`.

Things that will NOT be changed

• Variable names, constant names, function names, class names, struct names, etc.
• General code structure of Half-Life SDK

Proposals

• Can we move project files under projects/vs2019 to projects/? Or do we need to keep the file structure for compatibility?

[fel1x-developer]

Opened pull request #204