Use magic_enum for default enum value in help menu

[vlovero]

Currently the default value for enum types gets displayed as an integer. magic_enum can be used to make the help print out a little more clear.

[xerpi]

This would be really helpful!

[xerpi]

There's a warning:

[build] foo/build/debug/_deps/argparse-src/include/argparse/argparse.hpp:110:5: warning: control reaches end of non-void function [-Wreturn-type]
[build]   110 |     }
[build]       |     ^

The problem is that given

#ifdef HAS_MAGIC_ENUM
        } else if constexpr (std::is_enum<T>::value) {
            for (const auto &[name, value] : magic_enum::enum_entries<T>()) {
                if (v == name) {
                    return std::string{ value };
                }
            }
#endif
        } else if constexpr (has_ostream_operator<T>::value) {
            return static_cast<std::ostringstream &&>((std::ostringstream() << std::boolalpha << v)).str();       // https://github.com/stan-dev/math/issues/590#issuecomment-550122627
        } else {
            return "unknown";
        }

If (v == name) fails, then there's no return statement.
Maybe we should just move the return "unknown"; outside of the else.

[morrisfranken]

Hi @vlovero and @xerpi , thanks for the PR. Much appreciated! I've now merged this from pr #33