feat: Add stepIcon renderer

[kingwubin]

feat($step):增加precess状态下的进度特性，以百分比来展示，以及popover效果

变更点

step：steps的props里增加了progressPercentage的接口，在rc的step组件中进行调用，该函数返回reactNode，该接口的功能与progressDot类似，progressPercentage函数在antd中进行定义，在antd中，可以复用progress组件实现进度效果，进度的百分比也在antd中设置与实现；最外层div增加了的ant-steps-percentage的className，提供样式修改的能力；

增加的特性如图所示：

[codecov]

Codecov Report

Merging #118 into master will increase coverage by 6.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   91.78%   98.63%   +6.84%     
==========================================
  Files           2        2              
  Lines          73       73              
  Branches       25       25              
==========================================
+ Hits           67       72       +5     
+ Misses          6        1       -5     

Impacted Files	Coverage Δ
src/Step.tsx	97.77% <100.00%> (+9.40%)	⬆️
src/Steps.tsx	100.00% <100.00%> (+3.33%)	⬆️

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[yoyo837]

请添加测试用例

[zombieJ]

看了一下，这个 progress 似乎就是管 icon 的，建议类似 Table 的 expandable.expandIcon 一样直接叫做 stepIcon 属性更好懂。

[afc163]

标题和内容不符。