Add HasWinUI2 condition and update PackageReference for Microsoft.UI.Xaml

Author: Arlodotexe

MultiTarget/MultiTargetIdentifiers.props

@@ -63,5 +63,7 @@
     <WinUIMajorVersion Condition="'$(HasWinUI)' == 'true' AND '$(IsWinAppSdk)' == 'true' AND $(WinUIMajorVersion) == ''">3</WinUIMajorVersion>
 
     <UseUwp Condition="'$(HasWinUI)' == 'true' AND '$(IsUwp)' == 'true' AND '$(MultiTargetPlatformIdentifier)' == 'windows'">true</UseUwp>
+    
+    <HasWinUI2 Condition="'$(HasWinUI2)' == '' AND '$(HasWinUI)' == 'true' AND '$(WinUIMajorVersion)' == '2'">true</HasWinUI2>
   </PropertyGroup>
 </Project>

MultiTarget/PackageReferences/Uwp/Microsoft.UI.Xaml.props

@@ -1,12 +1,12 @@
 <Project>
 	<!-- Condition must be on a <When> statement in its own file for this to work in Visual Studio. https://stackoverflow.com/a/16557059 -->
 	<Choose>
-		<When Condition="'$(HasWinUI)' == 'true' AND '$(MultiTargetPlatformIdentifier)' != 'windows'" >
+		<When Condition="'$(HasWinUI2)' == 'true' AND '$(MultiTargetPlatformIdentifier)' != 'windows'" >
 			<ItemGroup>
 				<PackageReference Include="Microsoft.UI.Xaml" Version="2.8.6" />
 			</ItemGroup>
 		</When>
-		<When Condition="'$(HasWinUI)' == 'true' AND '$(MultiTargetPlatformIdentifier)' == 'windows'" >
+		<When Condition="'$(HasWinUI2)' == 'true' AND '$(MultiTargetPlatformIdentifier)' == 'windows'" >
 			<ItemGroup>
 				<!-- Prerelease version needed for TargetFramework support. Not needed by uap. -->
 				<PackageReference Include="Microsoft.UI.Xaml" Version="2.8.7-prerelease.241119001" />