Implement a parser and a renderer for the UniAst JSON syntax used by BlockNote editor

We're integrating the BlockNote WYSIWYG editor in XWiki by reusing its integration in Cristal. Unlike other WYSIWYG editors, BlockNote doesn't use HTML as input / output syntax. It uses its own custom syntax to represent the editable blocks of content. The Cristal project has integrated BlockNote and implemented a more generic UniAst syntax (that gets converted to / from the BlockNote's own syntax).

In my current implementation for XWIKI-23025 I'm using Markdown as input / output for the editor, because:

• XWiki has an extension that adds support for the Markdown syntax to the XWiki rendering (server-side)
• Cristal provides utilities to convert between Markdown and UniAst (client-side)

But this means my code needs to:

• depend on the Markdown extension, server-side
• bundle the Markdown utility code in the generated client-side code

It would be faster, simpler and with fewer chances of losing stuff in translation if we introduce a parser / renderer server-side for the UniAst syntax, i.e. to go directly from UniAst to XDOM instead of going through Markdown.