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

examples/nextjs/pages/ControlledUncontrolled/index.tsx

@@ -2,8 +2,6 @@ import React from 'react';
 
 import ReactFlow, {
   useReactFlow,
-  Background,
-  BackgroundVariant,
   Node,
   Edge,
   ReactFlowProvider,
@@ -11,6 +9,8 @@ import ReactFlow, {
   useEdgesState,
 } from '@react-flow/core';
 
+import Background, { BackgroundVariant } from '@react-flow/background';
+
 const defaultNodes: Node[] = [
   {
     id: '1',