Add public is_flags type trait

[jannejy]

There is no public API to check at compile time whether an enum has is_flags = true. The only options are accessing detail::has_is_flags (internal) or writing a custom SFINAE trait.

Use case

template <typename E>
std::optional<E> generic_enum_cast(std::string_view sv) {
    if constexpr (magic_enum::is_flags_v<E>) {
        return magic_enum::enum_flags_cast<E>(sv);
    } else {
        return magic_enum::enum_cast<E>(sv);
    }
}