feat(hooks): add useNodesInitialized

packages/core/src/hooks/useNodesInitialized.ts

@@ -0,0 +1,12 @@
+import { ReactFlowState } from '../types';
+import { useStore } from './useStore';
+
+const selector = (s: ReactFlowState) => Array.from(s.nodeInternals.values()).every((n) => n.width && n.height);
+
+function useNodesInitialized(): boolean {
+  const initialized = useStore(selector);
+
+  return initialized;
+}
+
+export default useNodesInitialized;