feat(module:modal): support for custom close icons

[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

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[netlify]

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Built with commit c69f907

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[codecov]

Codecov Report

Merging #4072 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4072      +/-   ##
=========================================
+ Coverage    92.4%   92.4%   +<.01%     
=========================================
  Files         506     506              
  Lines       10758   10759       +1     
  Branches     1940    1940              
=========================================
+ Hits         9941    9942       +1     
  Misses        402     402              
  Partials      415     415

Impacted Files	Coverage Δ
components/modal/nz-modal.module.ts	100% <ø> (ø)	⬆️
components/modal/nz-modal.component.ts	94.85% <100%> (+0.02%)	⬆️

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[wzhudev]

LGTM