Support showing constraints information besides the current size of a widget

## Problem

I use the code in https://docs.flutter.dev/ui/layout/constraints to demonstrate my issue.

In the examples below, it is not possible to know immediately the constraints that is applied on some widgets.
To find that information, we need to find in a nearest ancestor that provides the **Render object** tab or the widget that has constraints property.
It would be nice if by selecting any widget, we can answer immediately the question: "What is the constraints this widget received from its parent?"

## Expectation

I would expect something similar to **Flex explorer** tab:

首先在 DevTools 中复现 Flutter constraints 文档中的示例，然后检查现有的 Flex explorer 标签页，将其作为最接近的既定先例。当选择任意 widget 时，除了显示其当前大小，还能显示它从 parent 接收到的 constraints，即表示工作完成。

由索引模型根据 Issue 内容生成。