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

We previously implemented a UniAst parser and renderer, XRENDERING-788, which helped us go from XWiki syntax -> XDOM -> UniAst syntax -> BlockNote syntax and back. It turns out we actually don't need the UniAst intermediary level, so it would be faster and more reliable to go directly from XWiki syntax to BlockNote. See https://forum.xwiki.org/t/use-blocknote-syntax-directly-instead-of-going-through-cristals-uniast/18336 .