add support for Zod v4

[voxspox]

"Zod 4 fixes a number of long-standing design limitations of Zod 3 in one fell swoop, paving the way for several long-requested features and a huge leap in performance. It closes 9 of Zod's 10 most upvoted open issues. With luck, it will serve as the new foundation for many more years to come." – see also https://zod.dev/v4

Typescript types of zod v3 and v4 are not compatible, thus there could be a createZod4Plugin() to support migration.

I tried v4 successfully in my web application. In a first test i copied the files from formkit/packages/zod/src and changed the import to import type { z } from "zod/v4". Some types have been restructured, e.g. z.string().uuid() is now z.uuid() (see migration guide) but it was very easy and worked.