Allow completion items to add using directives

Author: davidwengier

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Formatting/RazorFormattingService.cs

@@ -241,7 +241,7 @@ public async Task<ImmutableArray<TextChange>> GetHtmlOnTypeFormattingChangesAsyn
             triggerCharacter: '\0',
             _csharpOnTypeFormattingPass,
             collapseChanges: true,
-            automaticallyAddUsings: false,
+            automaticallyAddUsings: true,
             validate: false,
             cancellationToken: cancellationToken).ConfigureAwait(false);