fix: add package.json > engines > node to “node.js” group

This is so that the upgrade is combined with travis and nvm

lib/config/definitions.js

@@ -313,7 +313,15 @@ const options = [
     description: 'Configuration specifically for `package.json`>`engines`',
     stage: 'packageFile',
     type: 'json',
-    default: {},
+    default: {
+      packageRules: [
+        {
+          packageNames: ['node'],
+          groupName: 'node.js',
+          lazyGrouping: false,
+        },
+      ],
+    },
     mergeable: true,
     cli: false,
   },