Add support for using Zod to validate environment variables #10

While having base type checking is fine, it should be good to allow libraries like a Zod to have more exhaustive validation.
There are tons of use cases for this like only allowing certain strings or more advanced parsing of strings/numbers.

While the validateFunction allows you to write custom functions, I'm not entirely sure if this will work with Zod out of the box, perhaps we'd want to make this Zod-first (though that would impact bundle sizes). If this works we'll just need to add the use cases as an example and in the documentation and hope it doesn't conflict with the built in validation.