fix(module: steps): add current support

[nuonuoge]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd-mobile/blob/master/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] 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
[x] No

Other information

[codecov[bot]]

Codecov Report

Merging #762 (5c4a833) into master (c184b27) will increase coverage by 0.09%. The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
+ Coverage   86.90%   87.00%   +0.09%     
==========================================
  Files         164      164              
  Lines        6637     6641       +4     
  Branches     1111     1107       -4     
==========================================
+ Hits         5768     5778      +10     
+ Misses        424      421       -3     
+ Partials      445      442       -3     

Impacted Files	Coverage Δ
components/steps/steps.component.ts	88.88% <83.33%> (+7.30%)	:arrow_up:
components/steps/step/step.component.ts	93.33% <96.66%> (-0.79%)	:arrow_down:

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