Fix to allow init in props in no net5 projects

allows non-verbose record decerlation syntax
Fix for "error CS0518: Predefined type
'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported"
see
https://stackoverflow.com/questions/64749385/predefined-type-system-runtime-compilerservices-isexternalinit-is-not-defined
https://developercommunity.visualstudio.com/t/error-cs0518-predefined-type-systemruntimecompiler/1244809

Author: Rosuavio

UnitTests/UnitTests.UWP/IsExternalInit.cs

@@ -0,0 +1,8 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Runtime.CompilerServices
+{
+    internal static class IsExternalInit { }
+}

UnitTests/UnitTests.UWP/UnitTests.UWP.csproj

@@ -174,6 +174,7 @@
     <Compile Include="Helpers\Test_StorageHelper.cs" />
     <Compile Include="Helpers\Test_StreamHelper.cs" />
     <Compile Include="Helpers\Test_DeepLinkParser.cs" />
+    <Compile Include="IsExternalInit.cs" />
     <Compile Include="Markdown\Parse\BoldTests.cs" />
     <Compile Include="Markdown\Parse\CodeTests.cs" />
     <Compile Include="Markdown\Parse\HeaderTests.cs" />