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

[Lineez]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

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].

What does the proposed API look like?

How i can help you?