Add Microsoft.VisualStudio.SDK.Analyzers as per https://devblogs.microsoft.com/visualstudio/checklist-for-writing-great-visual-studio-extensions/ - Warnings not fixed

ILSpy.AddIn/ILSpy.AddIn.csproj

@@ -51,6 +51,7 @@
     <PackageReference Include="Microsoft.VisualStudio.ComponentModelHost" Version="15.6.27413" />
     <PackageReference Include="Microsoft.VisualStudio.Editor" Version="15.6.27740" />
     <PackageReference Include="Microsoft.VisualStudio.LanguageServices" Version="2.4.0" />
+    <PackageReference Include="Microsoft.VisualStudio.SDK.Analyzers" Version="16.0.29" />
     <PackageReference Include="Microsoft.VisualStudio.Shell.15.0" Version="15.6.27413" />
     <PackageReference Include="Microsoft.VisualStudio.Shell.Interop.14.0.DesignTime" Version="14.3.26929" />
     <PackageReference Include="Microsoft.VisualStudio.Text.UI" Version="15.6.27740" />