Retrieve the resolving schema from a ZodPromise

[bolencki13]

Hey all -- I was playing around with ZodPromise and hoping to add this functionality in a PR. I checked out the CONTRIBUTING.md first and it asked I open an issue. So here I am....

ZodPromise is created with a second Zod type that is checked when the promise resolves. I'd like to be able to access the schema the ZodPromise was created with. For example

import { z } from 'zod';

const Foo = z.literal('foo')
const MyPromise = z.promise(Foo)

MyPromise.resolves() // should be a method that returns the Foo schema declared above

I'm not sure there is a way to do this at the moment and I'm open to suggestions on variable naming and such. If this is something that makes sense to PR I can draft something but looking for some feedback first. Thanks.