feat(module:steps): support customized starting index

[wzhudev]

close #1994

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: #1994

What is the new behavior?

Now user can specify the starting index.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #2021 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2021      +/-   ##
==========================================
+ Coverage   95.96%   96.01%   +0.05%     
==========================================
  Files         473      470       -3     
  Lines       11516    11429      -87     
  Branches     1535     1515      -20     
==========================================
- Hits        11051    10974      -77     
+ Misses        132      131       -1     
+ Partials      333      324       -9

Impacted Files	Coverage Δ
components/steps/nz-steps.component.ts	98.52% <100%> (+0.11%)	⬆️
components/modal/nz-modal.component.ts	96.17% <0%> (-0.04%)	⬇️
components/pagination/nz-pagination.component.ts	99.29% <0%> (-0.01%)	⬇️
components/tree-select/nz-tree-select.component.ts	98.65% <0%> (-0.01%)	⬇️
components/button/nz-button.module.ts	100% <0%> (ø)	⬆️
components/button/nz-button.component.ts	100% <0%> (ø)	⬆️
components/modal/nz-modal-control.service.ts	100% <0%> (ø)	⬆️
components/switch/nz-switch.module.ts	100% <0%> (ø)	⬆️
components/core/wave/nz-wave.directive.ts
components/core/wave/nz-wave.module.ts
... and 1 more

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[wzhudev]

We should merge this after Ant Design accepts a PR.