Goal

When editing a markdown file
I want to be able to embed content from another app
either as a preview or editable

Approach

• define a markdown syntax (preliminary)
• add parsing to markdown-it
• add a node type for previews
• render the previews when syntax is in the markdown
• ensure it serializes back to the same format
• allow removing a preview
• allow inserting a preview
• Revert Add interactive flag to reference widgets #5313 and move the interactive flag to the new node type

Describe alternatives you've considered
Define all links in the form of a node that can either be a simple link or a preview. However normal links are inline and previews would be block content.

Additional context

• Part of 🍱 Interactive widgets server#42550