[bug report] Why does the tree control output data in the order of data rather than in the order of selection when selecting the check box. Can I support selective sequential output?

### Element UI version
2.15.2

### OS/Browsers version
window10 / Google Chrome 91.0.4472.77

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/pkxz/pen/bGqjjeP

### Steps to reproduce
勾选"三级 1-1-2"，再勾选"三级 1-1-1",输出内容为"三级 1-1-1 三级 1-1-2"。

### What is Expected?
但是按照勾选顺序，应该输出为"三级 1-1-2 三级 1-1-1"

### What is actually happening?
实际输出内容为"三级 1-1-1 三级 1-1-2"