Add check-format npm script

extensions/vscode/package.json

@@ -445,6 +445,7 @@
     "pretest": "npm run compile",
     "lint": "eslint src --ext ts",
     "format": "prettier . --write",
+    "check-format": "prettier . --check",
     "test": "node ./out/test/runTest.js"
   },
   "devDependencies": {