Import .NET 6 property page targets path in iOS sdk targets (#12556)

* Import .NET 6 property page targets path in iOS sdk targets

* Fixes MSBuildExtensionsPath casing on Microsoft.iOS.Windows.Sdk.targets

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>
Co-authored-by: Emanuel Fernandez Dell'Oca <ema@xamarin.com>

dotnet/Microsoft.iOS.Windows.Sdk/targets/Microsoft.iOS.Windows.Sdk.targets

@@ -15,4 +15,10 @@
 
   -->
 
+  <PropertyGroup Condition="'$(XamarinIOSDesignTimeTargetsPath)' == ''">
+    <XamarinIOSDesignTimeTargetsPath>$(MSBuildExtensionsPath)\Microsoft\VisualStudio\Xamarin\iOS\Xamarin.iOS.DesignTime.targets</XamarinIOSDesignTimeTargetsPath>
+  </PropertyGroup>
+
+  <Import Project="$(XamarinIOSDesignTimeTargetsPath)" Condition="Exists('$(XamarinIOSDesignTimeTargetsPath)')" />
+
 </Project>