Custom block-level elements are a feature that allows editors to support the creation of unique block elements with specific styles or functionalities. These elements can include card-style hyperlinks, mind maps, kanban boards, and more. By providing the ability to customize blocks, plugins or extensions can be developed to meet specific user requirements. This enhances the scalability of the editor, allowing it to accommodate a wider range of use cases.

The advantage of custom blocks is that they are rendered within the editor itself, making them more lightweight compared to widgets. Widgets, on the other hand, are independent DOM contexts and can be rendered outside the editor, resembling a standalone web page. Custom blocks are typically designed to be used within the editor environment and may not function or render correctly outside of it.

By supporting custom blocks, an editor can provide flexibility, extensibility, and consistency to its users, allowing them to create rich and diverse content while maintaining a unified syntax and user experience.

自定义块可能需要考虑内部插入protyle的场景，比如叶归的白板，就是会在自定义块中渲染protyle。
由于目前的事件实现方式，导致在自定义块中渲染了protyle以后，交互基本上不可用。

暂时只是提出这一点，后续我增加全面一点的测试以后，再继续补充