refactor: overhaul package for Laravel 12 support and fix critical bugs

- Fix customMorphMap key format (keys should be class FQCNs, values aliases)
- Add morphedByMany override for inverse relationships
- Add initializeHasCustomMorphMap using Laravel trait convention
- Use static::class instead of __CLASS__ for late static binding
- Rewrite test suite with 18 tests including DB integration
- Add Laravel 12, PHP 8.1-8.4 support, drop Laravel 8/9
- Add PHPStan (max level), Laravel Pint, CI matrix
- Modernize phpunit.xml.dist for PHPUnit 10/11
- Update README, CHANGELOG, add CONTRIBUTING.md

change ownership

bump version

fix readme md