[xabuild] add reference to Newtonsoft.Json

Builds on CI were failing with:

    error MSB4018: The "XamarinTelemetry" task failed unexpectedly.
    error MSB4018: System.IO.FileNotFoundException: Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The system cannot find the file specified.
    error MSB4018: File name: 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'
    error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySessionSettings.GetFaultEventBucketFilterJson(List`1 bucketFilters)
    error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySessionSettings.ToString()
    error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySession.SerializeSettings()
    error MSB4018:    at Xamarin.Common.Tasks.Telemetry.GetSession(IBuildEngine4 buildEngine, String vsTelemetrySession) in D:\a\_work\1\s\src\MSBuild\Xamarin.Common.Tasks\Telemetry.cs:line 62
    error MSB4018:    at Xamarin.Common.Tasks.XamarinTelemetry.Execute() in D:\a\_work\1\s\src\MSBuild\Xamarin.Common.Tasks\XamarinTelemetry.cs:line 31
    error MSB4018:    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
    error MSB4018:    at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()
    error MSB4018: WRN: Assembly binding logging is turned OFF.
    error MSB4018: To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
    error MSB4018: Note: There is some performance penalty associated with assembly bind failure logging.
    error MSB4018: To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

I believe the issue is that `Newtonsoft.Json.dll` is not in xabuild's
output directory.

Let's add a `@(PackageReference)` to Newtonsoft.Json 13.0.1, which has
an assembly version of 13.0.0.0 like the error message says.

Author: jonathanpeppers

tools/xabuild/xabuild.csproj

@@ -14,6 +14,7 @@
   </PropertyGroup>
   <Import Project="..\..\Configuration.props" />
   <ItemGroup>
+    <PackageReference Include="Newtonsoft.Json" Version="$(NewtonsoftJsonPackageVersion)" />
     <PackageReference Include="System.Buffers" Version="4.5.1" />
     <PackageReference Include="System.Memory" Version="4.5.5" />
     <PackageReference Include="System.Collections.Immutable" Version="$(SystemCollectionsImmutableVersion)" />