feat: single method to comine .unwrap().unwrap()

[juliusmarminge]

Currently, if you have a schema with both optional and nullable, you have to call unwrap twice in order to get the base type. It would be nice to get it immediately.

I propose adding something like .nonnullable() (to match TypeScript's NonNullable<T> type) that does this in one shot:

Sandbox: https://stackblitz.com/edit/vitejs-vite-ursctp?file=src%2Fmain.ts

[colinhacks]

My current thinking on this is described here: #3141 (comment)

If Zod ever did implemented .nonNullable() it would actually wrap the current schema in another ZodNonNullable schema, instead of unwrapping, because it needs to work in conjunction with transforms, refinements, .or(z.undefined()). e.g.

z.string().or(z.null()).refine(()=>true).transform(val => Math.random() > 0.5 ? val : undefined).nonNullable()