add builder dependency

Author: jasongrout

{{cookiecutter.github_project_name}}/package.json

@@ -55,6 +55,7 @@
     "@jupyter-widgets/base": "^1.1.10 || ^2 || ^3 || ^4.0.0-beta.1"
   },
   "devDependencies": {
+    "@jupyterlab/builder": "^3.0.0-beta.7",
     "@phosphor/application": "^1.6.0",
     "@phosphor/widgets": "^1.6.0",
     "@types/expect.js": "^0.3.29",