Add recommended VS Code extensions

Add Visual Studio Code configuration file for recommended extensions.

Author: martincostello

.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+  "recommendations": [
+    "EditorConfig.EditorConfig",
+    "github.vscode-github-actions",
+    "ms-dotnettools.csharp"
+  ]
+}