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/_app.tsx

@@ -2,6 +2,13 @@ import React from 'react';
 import { useRouter } from 'next/router';
 import '../styles/globals.css';
 
+// Unfortunately this doesn't work because preconsruct clears the dist folder and there is
+// no way to hook into the watch process to copy the files back to the dist folder
+// import '@react-flow/core/dist/theme-default.css';
+
+// this is a workaround for testing the theme. See explanation above.
+// import '../styles/theme-default.css';
+
 const routes = [
   '/',
   '/Backgrounds',