Feat: support for modules
Allow to load external modules. Module would ve a grammar with one entrypoint, a model.
A grammar, in model calls, could specify a module instead of a local model. Woumd enable to define different grammar elements by community and reuse them.
Model could be xxx (local), file://xxx path to local file module , or http://xxx url to remote module
Modume calls (possibly recursive, module coumd usr module), woumd be prefixed in grammar by module identifier (a module namaspace, or unique module name)