add playground:build script

Author: RobinMalfait

packages/@headlessui-react/package.json

@@ -21,6 +21,7 @@
   },
   "scripts": {
     "playground": "next dev",
+    "playground:build": "next build",
     "test": "../../scripts/test.sh",
     "build": "../../scripts/build.sh",
     "lint": "../../scripts/lint.sh"