Add magic *_exists properties

[erikn69]

Summary

This adds *_exists model properties, similar to the *_count properties.

I also added a write_model_relation_exists_properties config to disable it.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update
• [ ] Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• [x] Existing tests have been adapted and/or new tests have been added
• [x] Update the README.md
• [x] Code style has been fixed via composer fix-style