refactor(background): move background to own package, replace example import

examples/nextjs/pages/Empty/index.tsx

@@ -3,9 +3,7 @@ import React, { MouseEvent, CSSProperties } from 'react';
 import ReactFlow, {
   addEdge,
   Controls,
-  Background,
   Node,
-  BackgroundVariant,
   Connection,
   Edge,
   useNodesState,
@@ -13,6 +11,8 @@ import ReactFlow, {
   ReactFlowInstance,
 } from '@react-flow/core';
 
+import Background, { BackgroundVariant } from '@react-flow/background';
+
 const onInit = (reactFlowInstance: ReactFlowInstance) =>
   console.log('flow loaded:', reactFlowInstance);
 const onNodeClick = (_: MouseEvent, node: Node) => console.log('click', node);