refactor(module:mention): refactor mention

[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: #2381

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #2510 into next will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             next    #2510      +/-   ##
==========================================
+ Coverage   95.82%   95.82%   +<.01%     
==========================================
  Files         491      491              
  Lines       12324    12328       +4     
  Branches     1673     1674       +1     
==========================================
+ Hits        11809    11813       +4     
  Misses        148      148              
  Partials      367      367

Impacted Files	Coverage Δ
components/mention/nz-mention-trigger.ts	100% <ø> (ø)
components/mention/nz-mention-suggestions.ts	100% <ø> (ø)
components/mention/nz-mention.module.ts	100% <100%> (ø)
components/mention/nz-mention.component.ts	99.42% <100%> (ø)

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