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/core/src/components/Edges/BaseEdge.tsx

@@ -32,7 +32,14 @@ export default ({
 
   return (
     <>
-      <path style={style} d={path} className="react-flow__edge-path" markerEnd={markerEnd} markerStart={markerStart} />
+      <path
+        style={style}
+        d={path}
+        fill="none"
+        className="react-flow__edge-path"
+        markerEnd={markerEnd}
+        markerStart={markerStart}
+      />
       {text}
     </>
   );