FrameworkElement.FindChildren does not work for Tabview

### Describe the bug

I previously had my own functions for getting all logical children of FrameworkElements but wanted to now change all my boilerplate code over to the easy to read Extension methods from here.
Sadly this does not work for my use case. I have a TabView in my UI and TabView.FindChildren() returns nothing because the TabViewItems collection is not being looked at.

### Steps to reproduce

```text
- Create a new WinUI3 project and add this library to it.
- Create a Top-Level TabView with some TabViewItems and put any other FrameworkElements into those.
- Call FindChildren() on the TabView.
- See no results being returned.
```

### Expected behavior

TabView.FindChildren() returns TabViewItems and their children.

### 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)
- [X] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [X] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [X] 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

2022

### Visual Studio Build Number

17.7.6

### Device form factor

Desktop

### Additional context

_No response_

### Help us help you

No, I'm unable to contribute a solution.

Beginne am Einstiegspunkt der FindChildren-Erweiterung und reproduziere den gemeldeten WinUI-3-Fall mit einer TabView der obersten Ebene, die TabViewItems und verschachtelte FrameworkElements enthält. Verfolge, wie logische untergeordnete Elemente aufgezählt werden, und überprüfe anschließend, dass TabViewItems und ihre Nachfahren von der bestehenden Reproduktion oder von relevanten Tests zurückgegeben werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.