Update beforeSlide prototype to match Typescript declarations

Fixes # (issue)

[ ] 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

[ ] My code follows the style guidelines of this project (I have run pnpm run lint)
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] New and existing unit tests pass locally with my changes (I have run pnpm run test:ci-with-server/pnpm run test)
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[ ] I have recorded any user-facing fixes or changes with pnpm changeset.
[ ] My changes generate no new warnings
[ ] Any dependent changes have been merged and published in downstream modules

FormidableLabs / nuka-carousel