Export instance of zod

[waptik]

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.