UITabBar

- [x]  iOS15后，设置UIBarAppearance无效果 

[Fixed](https://www.jianshu.com/p/8b37428bab92):
iOS15 需要设置scrollEdgeAppearance
```
standardAppearance = appearance
if #available(iOS 15.0, *) {
  scrollEdgeAppearance = appearance
} else {
   // Fallback on earlier versions
}
```
---