feat(module: nav-bar): add icon string support

[nuonuoge]

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-mobile/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

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage    85.9%   85.91%   +0.01%     
==========================================
  Files         209      209              
  Lines        6889     6894       +5     
  Branches      977      978       +1     
==========================================
+ Hits         5918     5923       +5     
  Misses        559      559              
  Partials      412      412

Impacted Files	Coverage Δ
components/nav-bar/nav-bar.module.ts	100% <100%> (ø)	⬆️
components/nav-bar/nav-bar.component.ts	100% <100%> (ø)	⬆️

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[3fuyu]

Your work is hard ! I want to discuss some detail with you, can you join us dingtalk group?

[nuonuoge]

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