Use directly the BlockNote syntax instead of going through UniAst

The main goal of UniAst was to provide a client-side AST for Cristal (equivalent of XDOM for XWiki), but this is mostly useful for Cristal backends that don't have a server-side rendering engine. XWiki does, so UniAst is slowing things down rather than helping. It would be better to use directly the BlockNote syntax in case of XWiki, for the BlockNote editor integration. This means implementing a parser and renderer for BlockNote, server-side. One benefit of this is that it will allow us to set block metadata, which is required in order to ensure compatibility with the XWiki syntax.