Make Node._flow a writeable property

This is needed so an existing node constructor that does:

   Object.assign(this,config);

works when it tries to replace this._flow with config._flow.

packages/node_modules/@node-red/runtime/lib/nodes/Node.js

@@ -38,7 +38,11 @@ function Node(n) {
         // Make this a non-enumerable property as it may cause
         // circular references. Any existing code that tries to JSON serialise
         // the object (such as dashboard) will not like circular refs
-        Object.defineProperty(this,'_flow', {value: n._flow, })
+        // The value must still be writable in the case that a node does:
+        //     Object.assign(this,config)
+        // as part of its constructure - config._flow will overwrite this._flow
+        // which we can tolerate as they are the same object.
+        Object.defineProperty(this,'_flow', {value: n._flow, enumerable: false, writable: true })
     }
     this.updateWires(n.wires);
 }