Allow importing code for constraints

[franky47]

This feature request may be a bit far-fetched, feel free to close if out of scope, though I might give it a go if you're interested.

I have some fields which I'd like to annotate with a /// @zod.regex() constraint. The content of the regex is defined somewhere else in the app, and needs to find its way to the generated file.

Currently, I'm copying the regex to the schema file, which is brittle as it exists in two places (and god knows it's easy enough to make mistakes in one regex, trying to keep two in sync is a footgun).

One way to make constraint extensions via documentation use external code would be to use rich-comments again, to define imports in the model.

Example API:

/// ```@zod
/// import { publicKeyRegex } from 'modules/crypto/box'
/// ```
model User {
  publicKey String /// @zod.regex(publicKeyRegex)
}

Generated code:

import * as z from 'zod'
import { publicKeyRegex } from 'modules/crypto/box'

export const userModel = z.object({
  publicKey: z.string().regex(publicKeyRegex)
})