Method is_boolean

example/json.hpp:287–290

        bool is_null() const noexcept { return kind() == value_kind::null; }
        bool is_boolean() const noexcept
        {
            return kind() == value_kind::boolean;
        }
        bool is_number() const noexcept { return kind() == value_kind::number; }
        bool is_string() const noexcept { return kind() == value_kind::string; }
        bool is_object() const noexcept { return kind() == value_kind::object; }