请求新增 spliter 面板分割组件

[transtone]

What problem does this feature solve?

拖动边框改变区块大小。

What does the proposed API look like?

<a-splitter :margin="20">
    <div slot="left-pane">
        <div>Some left content here.</div>
    </div>
    <div slot="right-pane">
        <div>Some right content here.</div>
    </div>
</a-splitter>

参考项目：

1. splitpanes
   https://github.com/antoniandre/splitpanes

2. viewui 面板分割：
   https://www.iviewui.com/components/split
   https://github.com/view-design/ViewUI/tree/master/src/components/split

3. vue-splitter:
   https://github.com/rmp135/vue-splitter

4. vue-split-pane:
   https://github.com/PanJiaChen/vue-split-pane

前一个已经关闭的类似需求：
#1153

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