create-app: add start-backend script to template

.changeset/sweet-ears-move.md

@@ -0,0 +1,11 @@
+---
+'@backstage/create-app': patch
+---
+
+Added `"start-backend"` script to root `package.json`.
+
+To apply this change to an existing app, add the following script to the root `package.json`:
+
+```json
+"start-backend": "yarn workspace backend start"
+```

packages/create-app/templates/default-app/package.json.hbs

@@ -7,6 +7,7 @@
   },
   "scripts": {
     "start": "yarn workspace app start",
+    "start-backend": "yarn workspace backend start",
     "build": "lerna run build",
     "build-image": "yarn workspace backend build-image",
     "tsc": "tsc",