Proposal:Remove `using namespace Microsoft::UI::Xaml;` in C++/WinRT file templates

### Title

Remove `using namespace Microsoft::UI::Xaml;` in C++/WinRT file templates

### Summary

The default C++/WinRT file templates should not include using namespace Microsoft::UI::Xaml; as it causes namespace collisions and violates C++ best practices

### Rationale

The current default template includes:
```cpp
using namespace winrt;
using namespace Microsoft::UI::Xaml;
```
This causes namespace collisions with the Windows Implementation Library (WIL, which is by default bundled with winui3 project templates) and other libraries like WRL.

```cpp
#include
```

steps to reproduce:

- Create a new WinUI 3 C++/WinRT project
- Add a WIL header to a .cpp file: #include
- Observe compilation errors due to namespace collision

### Scope

| Capability | Priority |
| :---------- | :------- |
| Solves namespace collision issue with WIL | Must |
| Remove the using statement | Could |
| Change it to `using namespace winrt::Microsoft::UI::Xaml;` | Could

### Important Notes

_No response_

### Open Questions

_No response_

Search the C++/WinRT file-template sources for the exact `using namespace Microsoft::UI::Xaml;` statement and inspect how those templates are tested or generated. Reproduce the issue by adding `#include ` to a generated WinUI 3 C++ project, then verify that generated files no longer cause the namespace collision.

Written by the indexing model from the issue text.