Method is_number

src/framework/io/json.cpp:203–205

      number_value_(other.number_value_),
      string_value_(other.string_value_) {
    if (other.array_value_) {
        array_value_ = std::make_unique<Array>(*other.array_value_);
    }
    if (other.object_value_) {
        object_value_ = std::make_unique<Object>(*other.object_value_);
    }