[Bug] Adding children to heading node not handled well

### Bug Description

https://github.com/user-attachments/assets/26be0e35-45b8-42dc-881c-8107da261aa7

After adding a child to the heading node, the child disappears.

### How to Reproduce

1. Run the example app
2. Add heading node
3. Create new node
4. Move this new node as a child to the previously created heading node.

### Expected Behavior

This is not well handled. I would suggest doing:
1. Make it possible to have children under heading. If the user wants it, why not make it possible.
2. Converting the "move action" to an action that moves the item under the heading (on the same level). This is how it is handled in the AppFlowy desktop app.

### Operating System

Linux/MacOS

### AppFlowy Editor Version(s)

5.1.0

### Screenshots

_No response_

### Additional Context

_No response_