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

Checklist

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

[barryvdh]

Hmm it could be confusing when they're not actually there. So not sure if we should make it false by default