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?

先阅读 issue #1610，然后跟踪 Layout Explorer 的交叉轴选择，并使用 baseline 为 null 的 Flex 重现切换到 baseline 时的错误。当可以选择并可视化 CrossAxisAlignment.baseline，且不会触发所报告的错误时，即表示完成。

由索引模型根据 Issue 内容生成。