🧪 [Experiment] Shimmer

### Approved from Discussion

https://github.com/CommunityToolkit/Labs-Windows/discussions/381

### Problem Statement

A way to indicate to the user that certain parts of the screen are loading.

### Overview

This experiment adds the following components:
- **Shimmer**: a loading indication control

### Using

You can try it out via the NuGet Packages here:

**UWP**
https://dev.azure.com/dotnet/CommunityToolkit/_artifacts/feed/CommunityToolkit-Labs/NuGet/CommunityToolkit.Labs.Uwp.Shimmer
**WinUI 3**
https://dev.azure.com/dotnet/CommunityToolkit/_artifacts/feed/CommunityToolkit-Labs/NuGet/CommunityToolkit.Labs.WinUI.Shimmer

Read more about [Preview Packages here](https://aka.ms/wct/wiki/previewpackages).

**Code**
```xml

```

**Result**

**Docs & samples**
https://github.com/CommunityToolkit/Labs-Windows/blob/main/components/Shimmer/samples

### Additional info
**TO DO**
- [ ] Add sample of using an ItemSelector to swap out a loading template with the loaded content.

**FUTURE**
- It'd be really cool to investigate if we could make this easier to use, though I think that'll have to be an 8.1 thing, and this is basically just the raw component that knows how to fill a space and shimmer, eh?
- Can we skip the XAML resource file and do something with a `ContentPresenter` or make it attachable to an UI element (like `AttachedShadow`?

### Implementation Requirements

- [X] Working Prototype
- [X] Feature Complete
- [X] Documentation
- [X] Samples
- [ ] Unit Tests
- [ ] Community Feedback / Usage Testimonies

### Tested Platforms

- [X] UWP
- [X] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Technical Review

- [ ] Accessibility Audit
- [ ] API/Naming Review
- [ ] Code Quality/Style
- [ ] Dependency Review
- [ ] Design/Style Review
- [ ] Final Approval

### Community Help?

Yes