chore: Bump version to 5.0.0

[PresentKim]

Summary

Bumps the version from 4.2.0 to 5.0.0. The refactor in #1 and #2 contains backward-incompatible changes, so this is a major release.

Backward-incompatible changes since 4.2.0

• __call / __callStatic are gone. Call styles that only worked through them no longer exist (for example everyAs(), or calling __map()), and unknown methods raise PHP's native "undefined method" error instead of BadMethodCallException.
• keyExists uses array_key_exists (a null value now counts as existing).
• reduceRight passes the original keys to the callback (integer keys were renumbered before).
• Callbacks that declare fewer than 3 parameters no longer receive the extra arguments.
• first / last / random return null on an empty array instead of raising a warning; fill / slice accept $end = null.

Not changed

• Only the version field of composer.json changes. No tag or release is created by this PR.