feat(module: stepper): stepper add ngModel

[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 #181 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   86.22%   86.24%   +0.02%     
==========================================
  Files         222      222              
  Lines        7142     7156      +14     
  Branches     1002     1002              
==========================================
+ Hits         6158     6172      +14     
  Misses        587      587              
  Partials      397      397

Impacted Files	Coverage Δ
components/stepper/stepper.component.ts	96.15% <100%> (+0.59%)	⬆️

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