Support for Zod 4 (now in beta)

[lawvs]

See colinhacks/zod#4074

[colinhacks]

Here's a dedicated guide for library authors that answers some common questions: https://v4.zod.dev/library-authors

1. Best practices around peer dependencies
2. How to support Zod v3 and Zod v4 simultaneously
3. How to support Zod & Zod Mini without extra work

Note that the @zod/core package has been abandoned in favor of a subpath: zod/v4/core. This makes it much easier for libraries with build on top of Zod with a single peer dependency on "zod". The reasons for this change are explained in more detail in the beta announcement: https://v4.zod.dev/v4