Better Avatar component

**Is your feature request related to a problem? Please describe.**
The current Avatar component(s) do the job, but they can do it better. Handling Avatar.Image and Avatar.Text is painful with if/else since it doesn't support fallback.

**Describe the solution you'd like**
A small refactor to the Avatar component to support Image mainly, and fallback to Text

Example:
```typescript

P

```

**Describe alternatives you've considered**
Manually checking for image loading, if it fails then I need to render Avatar.Text instead.

首先定位 Avatar 组件及其现有的 Avatar.Image 和 Avatar.Text 入口点。在实现所要求的 AvatarImage 和 AvatarFallback 用法之前，查看附近组件的测试；当图像不可用时能够显示文本并渲染图像，即表示完成。

由索引模型根据 Issue 内容生成。