Allow NuGet within the IDE

It appears fast enough to use now. 🎉

NuGet.config

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
 <configuration>
-  <packageRestore>
-    <!-- NuGet Restore was observed to cause performance problems for in-IDE builds,
-         so it has been disabled.
-
-         ⚠ THIS IS A PERFORMANCE-CRITICAL LINE. DO NOT REMOVE. ⚠ -->
-    <add key="enabled" value="false" />
-  </packageRestore>
   <packageSources>
     <clear />
     <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />