feat: add support for AsEnumCollection casts

[Braunson]

Summary

This adds support for the AsEnumCollection casts. Referenced in the Laravel documentation.

This references #1553

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)
• [ ] 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] Add a CHANGELOG.md entry
• [ ] Update the README.md
• [x] Code style has been fixed via composer fix-style

[Braunson]

Just a heads up @mfn the composer fix-style command is what is adding the strict types so I didn't commit any changes from it. If that's in the docs to run, you may want to either accept that change or update the php-cs-fixer rules.

[barryvdh]

Thanks!