Added Renovate to package.json

no issue

- the renovate.json file was removed to clean up the repository files,
  but we still want Renovate to update dependencies
- Renovate allows us to configure the config in the package.json, so
  this commit moves the previous contents there

package.json

@@ -88,5 +88,10 @@
                 "width": 2000
             }
         }
+    },
+    "renovate": {
+        "extends": [
+            "@tryghost:theme"
+        ]
     }
 }