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

packages/css-utils/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "@react-flow/css-utils",
+  "version": "11.0.0",
+  "description": "Utils for handling styles.",
+  "main": "dist/react-flow-css-utils.cjs.js",
+  "module": "dist/react-flow-css-utils.esm.js",
+  "license": "MIT"
+}