Refine `X4Attribute` to include the default constructible / assignable trait

[yaito3014]

One of the post-merge todo introduced by #13.

x4/include/iris/x4/core/attribute.hpp

Lines 34 to 42 in 3a697e4

	template<class T>
	concept X4NonUnusedAttribute =
	!X4UnusedAttribute<T> &&
	std::is_object_v<T> && // implies not reference
	!std::is_base_of_v<detail::parser_base, std::remove_const_t<T>> &&
	std::move_constructible<std::remove_const_t<T>>;
	// TODO: `fusion::iterator_range` does not satisfy these due to `fusion::vector`'s iterator being a reference type
	//std::default_initializable<std::remove_const_t<T>> &&
	//std::assignable_from<std::remove_const_t<T>&, std::remove_const_t<T>>;