TokenizingTextBox needs a GetIsAutoSuggestTextBoxFocused() method

### Describe the bug

There is no way to act on `TokenizingTextBox` `GotFocus` event handler to fill its `SuggestedItems` if its `AutoSuggestBox` got the focus.

Adding

**TokenizingTextBox.cs**

```
public bool GetIsAutoSuggestTextBoxFocused()
{
return !(GetFocusedElement() is TokenizingTextBoxItem);
}
```

will give us this possibility...

### Steps to reproduce

```text
s.o.
```

### Expected behavior

Being able to suggest the user items without having him to enter any character...

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [ ] 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.

Start in TokenizingTextBox.cs and inspect how focus is tracked between the control and its AutoSuggestBox. Verify the requested GetIsAutoSuggestTextBoxFocused() API against the existing focus behavior. Done means consumers can determine whether the AutoSuggestBox has focus and use that state to populate SuggestedItems.

Scritto dal modello di indicizzazione a partire dal testo della issue.