Reintroduce XAML Island Helpers

### Describe the bug

Like was written on the 8.0 release notes, Windows App SDK 1.4 indeed introduced XAML Island support, but only by adding `DesktopWindowXamlSource`. The Hosting API of [WinUI 3](https://learn.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.hosting?view=windows-app-sdk-1.4) looks comparable to [UWP](https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.hosting?view=winrt-22621). There are no convenience wrappers available around this class, like the `WindowsXamlHost` control from [Microsoft.Toolkit.Win32](https://github.com/CommunityToolkit/Microsoft.Toolkit.Win32). The absence of such control makes XAML Islands impractical for migrating WPF/WinForms apps to WinUI 3. Therefore I want to suggest to reconsider implementing `WindowsXamlHost`.

### Steps to reproduce

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### Expected behavior

Not applicable.

### Screenshots

_No response_

### Code Platform

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

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

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