refactor(module:alert): refactor alert

[vthinkxie]

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
[ ] 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
[ ] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #4779 into master will decrease coverage by 0.02%.
The diff coverage is 94.23%.

@@            Coverage Diff             @@
##           master    #4779      +/-   ##
==========================================
- Coverage   93.25%   93.23%   -0.03%     
==========================================
  Files         505      505              
  Lines       12734    12736       +2     
  Branches     1957     1955       -2     
==========================================
- Hits        11875    11874       -1     
- Misses        447      451       +4     
+ Partials      412      411       -1

Impacted Files	Coverage Δ
components/alert/alert.module.ts	100% <ø> (ø)
components/alert/alert.component.ts	94.23% <94.23%> (ø)
components/card/nz-card.component.ts
components/card/nz-card-grid.directive.ts
components/card/nz-card.module.ts
components/card/nz-card-meta.component.ts
components/card/nz-card-loading.component.ts
components/card/nz-card-tab.component.ts
components/card/card.module.ts	100% <0%> (ø)
components/card/card-meta.component.ts	100% <0%> (ø)
... and 6 more

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