Port Shadows

[niels9001]

### Base Porting Checklist
- [x] Update namespace (to `CommunityToolkit.WinUI.Shadows`)
- [x] Merge differences from UWP main to WinAppSDK
- [ ] Import original doc or setup new doc. Needs a refresh/update, and probably split
- [ ] Samples. Only setup StringFormatConverter test case, need others
- [ ] All brought over tests pass for UWP/WinAppSDK
- [ ] Add more tests for other converters not covered? (or at least well-used ones)
- [ ] Support for WASM via Uno Platform

## Notable Changes
- [ ] DropShadowPanel is deprecated - but a lot of (1P) apps use it. Any actions needed to make migration easier? Do we still port it?

## Future Improvements

[michael-hawker]

@niels9001 maybe we should take a quick look at where they're still using it and see if we can ascertain why that is and how we can help migrate them.

[michael-hawker]

These are technically extensions, so for now bringing them over with #6