3.0.0-beta1

fix: iterator return types and null safety checks
test: Modernize test suite
docs: Add upgrading guide for 3.0 src/
docs: Make type hints more correct
chore(hordeyml): Add keywords
chore: add phpunit.cache to gitignore file
style: php-cs-fixer on whole code base
test: Cover PR #10 with unit tests for different types
fix filterDefault for non-string values in mysql schema