fix: Button, DatePicker, Layout d.ts

[Pythonfo]

ButtonGroup 应该是 Button 的静态属性。

RangePicker, MonthPicker, WeekPicker 也是 DatePicker 上的静态属性。

从组件的源码上看来，Layout上除了Sider子组件为静态属性外，还有Header, Content, Footer

然后在 class 中，propertyName: type 的意思是这个属性是什么类型，而不是是什么。所以加上了 typeof。
e.g. without typeof，I coding

Vue.component(Form.Item.name, Form.Item)

intellisense show Form.Item is a vue instance, not class，but expect is a class.

[codecov]

Codecov Report

Merging #422 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #422   +/-   ##
=======================================
  Coverage   87.62%   87.62%           
=======================================
  Files         145      145           
  Lines        4744     4744           
  Branches     1404     1404           
=======================================
  Hits         4157     4157           
  Misses        543      543           
  Partials       44       44

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[tangjinzhou]

👍

[github-actions]

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