fix(module:mention): preserve white spaces when insert suggestions

[hsuanxyz]

fix #6175

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?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6505 (440a38b) into master (85ea273) will increase coverage by 0.03%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master    #6505      +/-   ##
==========================================
+ Coverage   89.82%   89.86%   +0.03%     
==========================================
  Files         478      480       +2     
  Lines       15314    15325      +11     
  Branches     2475     2334     -141     
==========================================
+ Hits        13756    13772      +16     
+ Misses        940      935       -5     
  Partials      618      618              

Impacted Files	Coverage Δ
components/pagination/pagination-item.component.ts	100.00% <ø> (ø)
components/rate/rate.component.ts	96.03% <ø> (ø)
components/space/space.module.ts	100.00% <ø> (ø)
components/mention/mention.component.ts	97.72% <75.00%> (+0.01%)	⬆️
components/mention/config.ts	100.00% <100.00%> (ø)
components/mention/mention-trigger.ts	100.00% <100.00%> (ø)
components/space/space-item.component.ts	100.00% <100.00%> (ø)
components/space/space-item.directive.ts	100.00% <100.00%> (ø)
components/space/space.component.ts	100.00% <100.00%> (ø)
components/tooltip/base.ts	91.98% <100.00%> (+0.94%)	⬆️
... and 4 more

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