[Tree-Select] Ask switcherIcon slot support and update docs

[Lineez]

Hello, thanks you for this great work.

I would like to have a support "switcherIcon" slot for [Tree-Select]. Right now we have only support for [Tree].

How i can help you?

[github-actions]

Hello @Lineez, your issue has been closed because it does not conform to our issue requirements. Please use the Issue Helper to create an issue, thank you!

你好 @Lineez，为了能够进行高效沟通，我们对 issue 有一定的格式要求，你的 issue 因为不符合要求而被自动关闭。你可以通过 issue 助手 来创建 issue 以方便我们定位错误。谢谢配合！