Fix undefined behavior (get is moved to magic_enum::containers)

[IvanArkhipov1999]

In magic_enum std::get is extended https://github.com/Neargye/magic_enum/blob/master/include/magic_enum/magic_enum_containers.hpp#L1130. But с++ documentation states, that "It is undefined behavior to declare a full specialization of any standard library function template." (https://en.cppreference.com/w/cpp/language/extending_std section "Function templates and member functions of templates").

So, get is moved to magic_enum::containers.

Closes #315

[Neargye]

Thanks!