Make the blocknote editor extensible

Currently, the blocknote-headless package does not allow to easily register editor extensions.
Examples of use of such an extension mechanism are:

• override the UI to select an image with a new modal (e.g., integration of xwiki's legacy)
• introduce a new "macro" block + the modals to select/edit macros
  Note: this extension mechanism must be agnostic of UI implementation frameworks such as Vue or React (i.e., a modal developed in jQuery or any other web technology must be embeddable)