C#: Set the extractor solution file as the default solution for the C# Dev Kit and add config for the Format usings plugin.

Author: michaelnebel

csharp/.vscode/settings.json

@@ -7,5 +7,8 @@
         "editor.defaultFormatter": "ms-dotnettools.csharp"
     },
     "omnisharp.enableEditorConfigSupport": true,
-    "omnisharp.enableRoslynAnalyzers": true
+    "omnisharp.enableRoslynAnalyzers": true,
+    "csharpFormatUsings.splitGroups": false,
+    "csharpFormatUsings.sortOrder": "Xunit System Microsoft Semmle.Util Semmle",
+    "dotnet.defaultSolution": "CSharp.sln"
 }