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/renderer/src/index.ts

@@ -0,0 +1,4 @@
+export * from '@react-flow/core';
+export * from '@react-flow/minimap';
+export * from '@react-flow/controls';
+export * from '@react-flow/background';

packages/renderer/src/theme-default.css

@@ -0,0 +1 @@
+@import '@react-flow/core/dist/theme-default.css';