feat(comp:select,comp:tree-select,comp:cascader): add selectedItem slot

[liuzaijiang]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added/updated or not needed
• Docs and demo have been added/updated or not needed

What is the current behavior?

What is the new behavior?

Other information

增加全选时只显示全选标签的demo

[idux-bot]

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[codecov]

Codecov Report

Merging #1257 (8b6f0bd) into main (ae21248) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1257      +/-   ##
==========================================
- Coverage   93.05%   93.04%   -0.01%     
==========================================
  Files         320      320              
  Lines       29448    29452       +4     
  Branches     3339     3340       +1     
==========================================
+ Hits        27403    27405       +2     
- Misses       2045     2047       +2     

Impacted Files	Coverage Δ
...ages/components/_private/selector/src/Selector.tsx	91.05% <50.00%> (-0.65%)	⬇️

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