fix: React custom blocks not valid drop targets (#1511)

* Made React custom blocks valid drop targets

* Removed commented out handler

Author: matthewlipski

packages/react/src/schema/ReactBlockSpec.tsx

@@ -71,6 +71,7 @@ export function BlockContentWrapper<
   return (
     // Creates `blockContent` element
     <NodeViewWrapper
+      onDragOver={(event: DragEvent) => event.preventDefault()}
       // Adds custom HTML attributes
       {...Object.fromEntries(
         Object.entries(props.domAttributes || {}).filter(