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

```text
Not applicable.
```

### 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_

### Help us help you

Yes, I'd like to be assigned to work on this item.

Beginne mit der Prüfung der Windows App SDK XAML Hosting API und des vorhandenen Microsoft.Toolkit.Win32 WindowsXamlHost-Steuerelements, auf das im Issue verwiesen wird. Lege den Umfang des Convenience-Wrappers und sein erwartetes Verhalten für die Migration von WPF und WinForms fest; als abgeschlossen gilt die Aufgabe, wenn ein unterstützter WindowsXamlHost-Helfer für WinUI 3 XAML Islands verfügbar ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.