Allow more flexible input for flex values

Currently the flex inputs in the Devtools UI only allows flex value inputs for `null` and `0` through `5`.
A fix is being put in that will show flex values >5 to show in the UI, but that will only be if the user has put in that flex value programmatically.

After the fix is put in, the flex input will look as follows, if a value of 70 is put in the flex box, and then user clicks on it.
Before Click
After Click

This approach still has issues:
- having such a long like is unwieldy from a UI perspective
- the user has no way to enter numbers > 5 from the UI.

To improve this experience we should find a way to support more diverse input and a UI experience.
Potentially we could have a text input, that would allow us to enter an integer, or null.

从 Devtools UI flex input 开始，复现 null 和 0 到 5 之间的值的当前行为，包括以编程方式输入的大于 5 的值。在更改之前，先定义支持的输入和交互；完成标准是用户可以通过 UI 输入更大范围的 flex 值，同时显示的长值不会变得难以处理。

由索引模型根据 Issue 内容生成。