Disable implicit namespace imports (#4846)

* Disable implicit namespace imports.  WPF will not build until this is explicitly supported.

Author: ryalanms

src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/Microsoft.WinFX.targets

@@ -18,6 +18,9 @@
     <AlwaysCompileMarkupFilesInSeparateDomain Condition="'$(BuildingInsideVisualStudio)' == 'true' and '$(AlwaysCompileMarkupFilesInSeparateDomain)' == ''">true</AlwaysCompileMarkupFilesInSeparateDomain>
     <AlwaysCompileMarkupFilesInSeparateDomain Condition="'$(AlwaysCompileMarkupFilesInSeparateDomain)' == '' ">true</AlwaysCompileMarkupFilesInSeparateDomain>
     <LocalizationDirectivesToLocFile Condition="'$(LocalizationDirectivesToLocFile)' == ''">None</LocalizationDirectivesToLocFile>
+
+    <!-- Disable implicit namespace imports for WPF.  This should be removed after support is added in .NET 6.0 Preview 8. -->
+    <DisableImplicitNamespaceImports Condition="'$(DisableImplicitNamespaceImports)' != 'false'">true</DisableImplicitNamespaceImports>
   </PropertyGroup>
 
   <!-- Some Default Settings -->