make sandbox plugin private (#746)

Author: karthikjeeyar

workspaces/sandbox/plugins/sandbox/package.json

@@ -4,6 +4,7 @@
   "main": "src/index.ts",
   "types": "src/index.ts",
   "license": "Apache-2.0",
+  "private": true,
   "publishConfig": {
     "access": "public",
     "main": "dist/index.esm.js",