Add support for "custom" definitions about zod-to-json-schema

[StefanTerdell]

@bacebu4 Check the tests pr #28, let me know what you think

[StefanTerdell]

Released as 3.19.2

[StefanTerdell]

I like the idea in principle, but I see a few issues:

1. .brand() has a pretty substantial effect on your Zod schemas by essentially disabling duck-typing
2. It would require a substantial refactoring of the current reference collection flow

How about something like this?

const myRecurringSchema = z.string()
const myObjectSchema = z.object({a: myRecurringSchema, b: myRecurringSchema})

const myJsonSchema = zodToJsonSchema(myObjectSchema, { definitions: { myRecurringSchema }})

With output

{
  "type": "object",
  "properties": {
    "a": {
      "$ref": "#/definitions/myRecurringSchema"
    },
    "b": {
      "$ref": "#/definitions/myRecurringSchema"
    }
  },
  "definitions": {
    "myRecurringSchema": {
      "type": "string"
    }
  }
}

This way we limit the blast radius by avoiding branding and can instantiate a References object "preloaded" with ref paths using the keys from the definitions option record

[bacebu4]

@bacebu4 Check the tests pr #28, let me know what you think

Really liked the implementation, much better and cleaner than proposed solution