docs: add the selection attribute to handleDrop (#332)

将 selection 属性添加至默认编辑器的 handleDrop 扩展中。相关改动查看 halo-dev/halo#5570

/kind documentation

```release-note
None
```

docs/developer-guide/plugin/api-reference/ui/extension-points/default-editor-extension-create.md

@@ -408,12 +408,14 @@ export interface DraggableItem {
  slice,
  insertPos,
  node,
+ selection,
  }: {
  view: EditorView;
  event: DragEvent;
  slice: Slice;
  insertPos: number;
  node: Node;
+ selection: Selection;
  }) => boolean | void;
  allowPropagationDownward?: boolean; // 是否允许拖拽事件向内部传播，
 }