Add MSBuild glob for Windows Assets

[jfversluis]

Description of Change

Similar to #23269 but specific to Windows Assets.

This adds a glob that will take any asset under Platforms\Windows\Assets\** and put that in the app bundle root.

Potentially breaking change for people as one workaround was to just reference the file by specifying the whole path, for example: <Image Source="{OnPlatform flag.png, WinUI=Platforms/Windows/Assets/Images/flag.png}" />

Issues Fixed

Fixes #16755

[mattleibow]

Why the Images sub folder? On android and iOS you would just stick things in the Platforms/Android/Resources/drawable (Android native convention) or Platforms/iOS/Resources (just in the folder)

For Windows, do we have any common practices that Images folder is important? What about non-image things?

I know I also did this here: https://github.com/dotnet/maui/blob/main/src/Templates/src/templates/maui-multiproject/MauiApp.1.WinUI/MauiApp.1.WinUI.csproj#L33

But, we I right? I just did that for the template.

What happens if I add a Platforms/Windows/Assets/?

I see this also has that pattern: https://learn.microsoft.com/en-us/windows/uwp/app-resources/tailor-resources-lang-scale-contrast

But, is there a guide doc or something?

[jfversluis]

• Do we want to make this specific to the Assets/Images folder?
• How do subfolders behave on other platforms?