add global environment variable feature

packages/node_modules/@node-red/nodes/locales/en-US/messages.json

@@ -1123,5 +1123,8 @@
         "warn": {
             "nonumber": "no number found in payload"
         }
+    },
+    "global-config": {
+      "tip": "<p>This node holds global configuration of flows.</p><p>Do not delete this node unless you understand underlying mechanism.</p>"
     }
 }