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/FloatingEdges/index.tsx

@@ -1,15 +1,15 @@
 import React, { useCallback } from 'react';
 
-import ReactFlow, {
+import {
+  ReactFlow,
+  Background,
   addEdge,
   ReactFlowInstance,
   EdgeTypes,
   Connection,
   useNodesState,
   useEdgesState,
-} from '@react-flow/core';
-
-import Background from '@react-flow/background';
+} from '@react-flow/renderer';
 
 import styles from './style.module.css';