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

examples/nextjs/pages/MultiFlows/index.tsx

@@ -2,7 +2,6 @@ import React, { FC } from 'react';
 
 import ReactFlow, {
   addEdge,
-  Background,
   Node,
   Edge,
   Connection,
@@ -12,6 +11,8 @@ import ReactFlow, {
   MarkerType,
 } from '@react-flow/core';
 
+import Background from '@react-flow/background';
+
 import styles from './multiflows.module.css';
 
 const initialNodes: Node[] = [