feat(module:tree-select): add the public methods

[hsuanxyz]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #3335 into master will increase coverage by <.01%.
The diff coverage is 97.5%.

@@            Coverage Diff             @@
##           master    #3335      +/-   ##
==========================================
+ Coverage   97.44%   97.44%   +<.01%     
==========================================
  Files         606      607       +1     
  Lines       11931    11935       +4     
  Branches      869      871       +2     
==========================================
+ Hits        11626    11630       +4     
  Misses        196      196              
  Partials      109      109

Impacted Files	Coverage Δ
components/tree/nz-tree.component.ts	99.25% <100%> (+0.5%)	⬆️
components/core/tree/public-api.ts	100% <100%> (ø)	⬆️
components/tree-select/nz-tree-select.component.ts	99.47% <100%> (-0.01%)	⬇️
components/core/tree/nz-tree-base.ts	96.87% <96.87%> (ø)

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