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

examples/nextjs/pages/Edges/index.tsx

@@ -1,7 +1,6 @@
 import React, { MouseEvent } from 'react';
 import ReactFlow, {
   addEdge,
-  Background,
   Connection,
   Controls,
   Edge,
@@ -13,6 +12,8 @@ import ReactFlow, {
   useNodesState,
 } from '@react-flow/core';
 
+import Background from '@react-flow/background';
+
 import MiniMap from '@react-flow/minimap';
 
 import CustomEdge from './CustomEdge';