Add support for Wagtail's ParentalKey relationship

[kevinhowbrook]

This patch adds compatibility with Wagtail CMS by:

1. Adding ParentalKey to the RELATIONSHIP_MAP with one-to-many cardinality
2. Modifying field type detection to use the field's class name for relationship mappings
3. Adding graceful error handling for unsupported relationship types

This allows django-diagram to work with Wagtail page models that extensively use ParentalKey relationships for their hierarchical structure, while maintaining backward compatibility with standard Django models.

Error without this patch when running:

    self.process_relationship(
  File "/venv/lib/python3.12/site-packages/django_diagram/app.py", line 56, in process_relationship
    ) = self.RELATIONSHIP_MAP[field_type]
        ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
KeyError: 'ParentalKey'