Improve incremental build performance

Author: drewnoakes

src/Directory.Build.props

@@ -3,6 +3,8 @@
   <PropertyGroup>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
+    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
+    <AccelerateBuildsInVisualStudio>true</AccelerateBuildsInVisualStudio>
   </PropertyGroup>
 
   <ItemGroup>