Merge: modelize_property: Add `unsafe_add_mclassdef2nclassdef`

Add a way to associate a `nclassdef` with its `mclassdef` without verification.

Currently, to attach an AClass with an MClass, you must use the `build_property` method which performs all the checks and builds the representation in the model. Thanks to the Nitbuilder which facilitates the construction of entities, we find ourselves in a case where we can build the representation of a class in the model and then build the corresponding ast. It is therefore necessary to provide a service to associate the both elements.

Pull-Request: #2817