[Bug Report] 常用功能：tree组件的插槽内，应该暴露出来节点的索引， 引用即可复现，目前没有索引
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- Dominant language
- Vue
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- 54k
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Description
### Element UI version
2.15.3
### OS/Browsers version
windows/chrom
### Vue version
2.6.10
### Reproduction Link
https://elementui.github.io/issue-generator/#/zh-CN
### Steps to reproduce
引用即可复现，目前没有索引
### What is Expected?
暴露索引
### What is actually happening?
引用即可复现，目前没有索引
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Assessment
This issue has not been assessed yet.