Port WebViewExtensions from 7.x

**Source**: Microsoft.Toolkit.Uwp.UI (7.x)
**Destination**: CommunityToolkit.WinUI.Extensions (8.x) — not yet ported

## Description

The WebViewExtensions class allows attaching HTML content to a WebView control through XAML directly or through a binding.

## Prerequisites

- [ ] WebView2 integration approach determined (see [#6 comment](https://github.com/CommunityToolkit/Windows/issues/6#issuecomment-1480282848))

## Porting Basics

- [ ] Namespace updated to `CommunityToolkit.WinUI.*`
- [ ] Code for UWP, WinUI3 and Uno Platform consolidated
- [ ] Desired MultiTargets configured
- [ ] Sample code migrated
- [ ] Concept docs migrated
- [ ] Tests migrated and passing

## Quality Evaluation

See [Porting Criteria](https://github.com/CommunityToolkit/Windows/wiki/Component-Quality-Evaluation) for detailed evaluation guidance.

- [n/a] UX: Accessibility reviewed
- [n/a] UX: Visual design reviewed
- [n/a] UX: Interaction patterns appropriate
- [ ] DX: API surface reviewed
- [ ] DX: API flexibility appropriate
- [ ] DX: Documentation updated
- [ ] MX: Breaking changes documented
- [ ] MX: Domain isolation appropriate
- [ ] MX: Test coverage adequate

## Tracking Gaps

*No gaps identified — porting work not yet started.*

## Closing

- [ ] All Porting Basics complete
- [ ] Quality Evaluation reviewed
- [ ] Gaps filed as sub-issues

---

## 7x Documentation Inventory

**Docs ported to 8x**: n/a

**Docs not in 8x**: n/a

## 7x Source Inventory

**Source already ported to 8x**: n/a

**Source not ported to 8x**: n/a

## Notes

- Not currently planned for porting. Open to community interest.
- **Technical requirement**: Would require WebView2 changes if ported.
- **Archive redirect**: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/windows/archive/webviewextensions