Refactor: CSS handling (#2344)

* refactor(css): only load base styles, add css task, cleanup exports
* style(base): add edge label bg
* refactor(css): use css-utils
* feat(core): add Panel component
* refactor(background): cleanup
* refactor(css-handling): cleanup

examples/nextjs/pages/UpdatableEdge/index.tsx

@@ -1,5 +1,6 @@
 import React, { useState, useCallback } from 'react';
-import ReactFlow, {
+import {
+  ReactFlow,
   updateEdge,
   addEdge,
   applyNodeChanges,
@@ -13,7 +14,7 @@ import ReactFlow, {
   HandleType,
 } from '@react-flow/core';
 
-import Controls from '@react-flow/controls';
+import { Controls } from '@react-flow/controls';
 
 const initialNodes: Node[] = [
   {