Parser does not emit all events for extension statements

This was detected during development of YANGTOOLS-1338. EffectiveStatementSupport objects registered for the FULL_DECLARATION phase do not see the full lifecycle of their corresponding statement.

They see onStatementAdded() first and then onFullDefinitionDeclared(), but not onStatementDefinitionDeclared() or any other of the intermediate callbacks.

This is understandable from how the reactor operates, but it is not documented anywhere and the integration surface does not provide any guidance which would steer implementers – all they see is their method not invoked.

We need to document this somewhere at the very least, but should also explore what we can do to at least warn of this caveat, if not actively prevent it from being possible.