Move Blocknote "headless" out of the Cristal monorepo

Having blocknote "headless" in the Cristal monorepo is a risk for the decoupling of this package with the rest of Cristal.
This is important since this component is expected to be Cristal agnostic and must be usable in various contexts such as XWiki too (but conceptually, not exclusively).
Therefore, we need to find a way to move this component in its repository while preserving a nice development experience as well as a good continous integration.