feat(module: stepper): 修复输入框输入显示问题

[shuizhongxiong]

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?

[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?

手动输入除数字之外的字符，显示值会变为NaN

Issue Number: 787、771

What is the new behavior?

1、输入框增加只能输入数字的限制
2、输入框增加小数和负数的输入判断
3、输入框增加 blur 时，对输入值进行有效判断并回填
4、增加相应测试用例
5、增加 demo 示例

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #791 (c18680a) into master (1d9a4a6) will decrease coverage by 0.08%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
- Coverage   86.99%   86.91%   -0.09%     
==========================================
  Files         164      164              
  Lines        6645     6677      +32     
  Branches     1109     1115       +6     
==========================================
+ Hits         5781     5803      +22     
- Misses        421      428       +7     
- Partials      443      446       +3     

Impacted Files	Coverage Δ
components/stepper/stepper.component.ts	90.85% <71.42%> (-6.12%)	⬇️
...ts/input-item/custom-input/custom-input.service.ts	87.09% <0.00%> (+3.22%)	⬆️

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