Fix devcontainer extensions key (#24359)

* Fix devcontainer vscode extensions definition

* Fix formatting

---------

Co-authored-by: Travis Plunk <travis.plunk@microsoft.com>

.devcontainer/devcontainer.json

@@ -9,6 +9,8 @@
   // Uncomment the next line to run commands after the container is created.
   "postCreateCommand": "cd src/powershell-unix && dotnet restore",
 
+  "customizations": {
+    "vscode": {
       "extensions": [
         "ms-azure-devops.azure-pipelines",
         "ms-dotnettools.csharp",
@@ -16,4 +18,6 @@
         "DavidAnson.vscode-markdownlint",
         "vitaliymaz.vscode-svg-previewer"
       ]
+    }
+  }
 }