[X] Consider producing build errors when XAML compilation is bypassed and XAML parsing at runtime is disabled

[simonrozsival]

The motivation is to prevent hitting XAML parsing codepaths at runtime when it is disabled and these codepaths would throw runtime exceptions (see #19310):

• disabling XAML compilation using <?xaml-comp compile="false" ?> should cause a build failure
• [XamlCompilation(XamlCompilationOptions.Skip)] on any class, module or assembly should cause build failure
• setting $(MauiXamlCValidateOnly)=true in Release should cause build failure
• any binding which can't be compiled should cause build failure
  • this has been partially covered by [X] Warn on non compiled bindings #19360 but we would also need to make sure all bindings with relative binding sources can be compiled (currently not implemented AFAIK)

If there are other cases in which it is possible to skip XAML compilation, they should cause build failures as well.

/cc @StephaneDelcroix

[simonrozsival]

• setting $(MauiXamlCValidateOnly) should fail only in Release or when $(IsTrimming)=true