Warn when any types of item conditions are used in legacy projects

[nkolev92]

MSBuild conditions on PackageReference or ProjectReference don't really work in VS for legacy projects, so it might be a good idea to warn whenever these conditions are seen.

We have the capability to warn on the commandline, but this isn't something that's possible in all implementations.

Example, the following condition will not be respected in VS, but will be respected in the commandline.

<ItemGroup>
   <PackageReference Include="Newtonsoft.Json" Version="13.0.1" Condition=" '$(ShouldReferenceNJ)' == 'true' " /> 
</ItemGroup>

See dotnet/msbuild#8889
https://developercommunity.visualstudio.com/t/visual-studio-manage-nuget-packages-ui-does-not-re/721485