x:Bind with function binding requires the binding source to be redeclared in idl in C++winrt

### Describe the bug

In winui3, `x:Bind` binding to an object's property is now fixed. The binding source does NOT need to be redeclared in idl. (This is an issue dates back from UWP)

However, `x:Bind` binding to an object's **method** still requires redeclaring it in idl (otherwise fails to build), which is inconsistent and causes confusion.

### Why is this important?

This is a xaml codegen issue in C++WinRT

### Steps to reproduce the bug

1. Create a new winui3 C++ project
2. Create a ViewModel with this idl. Implement the method with stub value
```
runtimeclass ViewModel
{
ViewModel();

Int32 MyProperty;
void Print();
}
```
3. In `MainWindow.xaml`, use this code
```xml

```

### Actual behavior

Build error:
```
C:\Users\Peter\Desktop\WinUI-Bug\35-xBindToFunction\Generated Files\MainWindow.xaml.g.hpp(108,51): error C2039: 'ViewModel': is not a member of 'winrt::_35_xBindToFunction::MainWindow'
01:54:31:318 1> C:\Users\Peter\Desktop\WinUI-Bug\35-xBindToFunction\Generated Files\MainWindow.xaml.g.hpp(110,51): error C2039: 'ViewModel': is not a member of 'winrt::_35_xBindToFunction::MainWindow'
```

You have to redeclare `ViewModel` in `MainWindow.idl` like this
```idl
runtimeclass MainWindow : Microsoft.UI.Xaml.Window
{
MainWindow();

ViewModel ViewModel{ get; };
}
```

### Expected behavior

It should not needing to redeclare it in idl. Like this property binding
```xml

```

### Screenshots

_No response_

### NuGet package version

2.3.0

### Windows version

_No response_

### Additional context
[Repro here](https://github.com/HO-COOH/WinUI-Bug/tree/main/35-xBindToFunction)

Reproduce the issue with the linked 35-xBindToFunction project, then inspect MainWindow.xaml, MainWindow.idl, and the generated MainWindow.xaml.g.hpp around the reported errors. Compare method binding with the working property binding; done means the method binding builds without redeclaring ViewModel in MainWindow.idl.

Written by the indexing model from the issue text.