feat: add zod 4 to peerDependencies

[marcalexiei]

https://github.com/colinhacks/zod/releases/tag/v4.0.1

Library authors — if you've already implemented Zod 4 support according to the best practices outlined in the Library authors guide, bump your peer dependency to include zod@^4.0.0:

// package.json
{
  "peerDependencies": {
    "zod": "^3.25.0 || ^4.0.0"
  }
}

There should be no other code changes necessary. No code changes were made between the latest 3.25.x release and > 4.0.0. This does not require a major version bump.