CrossAxisAlignment.baseline is missing from cross axis selection options in Layout Explorer

During my internship, I didn't get to implement the logic for visualizing CrossAxisAlignment.baseline because more often than not the baseline of the Flex widget would be null and triggering the change to baseline would trigger an error.

This issue is related to #1610, once #1610 is resolved, we can consider adding CrossAxisAlignment.baseline on the baseline?

Start by reading issue #1610, then trace the Layout Explorer's cross-axis selection and reproduce the error when switching to baseline with a Flex whose baseline is null. Done means CrossAxisAlignment.baseline can be selected and visualized without triggering the reported error.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.