fix(module:mention): getMention method should kepp the mention prefix

[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
[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?

没有保留前缀，无法区分类型。

getMention("... @vthinkxie ... #1142") // ["vthinkxie",  "1142"]

Issue Number: N/A

What is the new behavior?

保留前缀，能够区分类型。

getMention("... @vthinkxie ... #1142") // ["@vthinkxie",  "#1142"]

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #1287 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1287   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files         191      191           
  Lines        7926     7926           
  Branches     1041     1041           
=======================================
  Hits         7709     7709           
  Misses         25       25           
  Partials      192      192

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