feat(module:dropdown): display arrow for content dropdown

[Nicoss54]

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?

Drowpdown component miss the displaying of an arrow on the contend dropdown menu

Issue Number: N/A

What is the new behavior?

Dropdown component displayed an arrow on the content dropdown menu as the andt-design do:

https://ant.design/components/dropdown#dropdown-demo-arrow-center

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.13%. Comparing base (b0a4287) to head (56d63af).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #9329    +/-   ##
========================================
  Coverage   92.12%   92.13%            
========================================
  Files         561      561            
  Lines       19631    19650    +19     
  Branches     3107     3005   -102     
========================================
+ Hits        18086    18105    +19     
  Misses       1206     1206            
  Partials      339      339            

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[Laffery]

LGTM, I'd left a small suggestion on the showcase code

[Nicoss54]

@Laffery @HyperLife1119 could you paid attention to the trad, cause i used google trad and i'm not sure about the traduction. Really sorry for that

[Laffery]

@Laffery @HyperLife1119 could you paid attention to the trad, cause i used google trad and i'm not sure about the traduction. Really sorry for that

they are nice

[HyperLife1119]

LGTM