refactor(module:mention): refactor

[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
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] 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 #4755 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #4755   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         506      506           
  Lines       12776    12776           
  Branches     2015     2015           
=======================================
  Hits        11914    11914           
  Misses        451      451           
  Partials      411      411

Impacted Files	Coverage Δ
components/mention/mention-trigger.ts	100% <ø> (ø)
components/mention/mention.module.ts	100% <ø> (ø)
components/mention/mention.service.ts	90% <ø> (ø)
components/mention/mention-suggestions.ts	100% <ø> (ø)
components/mention/mention.component.ts	98.79% <100%> (ø)

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