Support for a z.example() method

[erunion]

We're open to PR'ing this in but before we do I'm curious if there's interest in a .example() method that like .describe() adds an example property into the ZodTypeDef interface so that tooling like zod-to-json-schema could generate JSON Schema representations with examples.

For example:

const documentedURL = z
  .url()
  .example('https://example.com')
documentedURL.example; // https://example.com

[ryceg]

This would be a great feature.

[erunion]

I have added support for this in #3518.