feat(module:mention): add prefix property to nzOnSearchChange payload

[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?

Issue Number: N/A

nzOnSearchChange: EventEmitter<string>

What is the new behavior?

export interface MentionOnSearchTypes {
  value: string;
  prefix: string;
}

nzOnSearchChange: EventEmitter<MentionOnSearchTypes>

Does this PR introduce a breaking change?

[X] Yes
[ ] No

Other information

类似 Github 的提及，@ 和 # 会请求不同的接口获取数据。需要在事件中带上触发的前缀才能识别。

[codecov]

Codecov Report

Merging #1296 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1296      +/-   ##
==========================================
+ Coverage   97.26%   97.26%   +<.01%     
==========================================
  Files         191      191              
  Lines        7926     7927       +1     
  Branches     1041     1041              
==========================================
+ Hits         7709     7710       +1     
  Misses         25       25              
  Partials      192      192

Impacted Files	Coverage Δ
components/mention/mention.component.ts	99.39% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c3b9c39...6fb7f53. Read the comment docs.