Put `GridResizeDirection` and `GridResizeBehavior` directly in the `CommunityToolkit.WinUI` namespace

[Arlodotexe]

Discussed in CommunityToolkit/Labs-Windows#706

^{Originally posted by 0x5bfa August 7, 2025}
As the title describes, we should put them in the top-level namespace since putting enums inside a class make them harder to be discovered.

[michael-hawker]

Are these used often outside of XAML which auto-knows what they are?