feat(module:skeleton): add nz-skeleton-element

[wenqi73]

Close #4619

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

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #4859 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4859      +/-   ##
==========================================
+ Coverage   92.94%   92.94%   +<.01%     
==========================================
  Files         505      506       +1     
  Lines       12741    12750       +9     
  Branches     1972     1965       -7     
==========================================
+ Hits        11842    11851       +9     
  Misses        474      474              
  Partials      425      425

Impacted Files	Coverage Δ
components/skeleton/skeleton.component.ts	100% <ø> (ø)
components/skeleton/skeleton-element.component.ts	100% <100%> (ø)
components/skeleton/skeleton.module.ts	100% <100%> (ø)
...omponents/date-picker/abstract-picker.component.ts	94.96% <0%> (-0.11%)	⬇️
components/cascader/cascader.component.ts	95.39% <0%> (ø)	⬆️
components/date-picker/date-picker.service.ts	100% <0%> (ø)	⬆️
components/tree-select/nz-tree-select.module.ts
components/tree-select/nz-tree-select.component.ts
components/tree-select/nz-tree-select.service.ts
... and 8 more

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