Export instance of zod#37

This library uses a locked version of zod which is `v3.21.4`. When a user uses a new version to use as schema, the editor(eg. vscode) points out a type definition error when you hover to the section in question so you are forced to downgrade your version and notify `deno-lint` to ignore that error. So i'd suggest that this library also export the instance of `zod` being used for users to use that instead in order to prevent such situations.