Support custom errors

[BTOdell]

https://zod.dev/error-customization?id=the-error-param

This library currently hardcodes the error string if parsing/coercion fails. Unfortunately, it would be a breaking change to make the z* validators functions instead of objects:

zPlainDate("error message")

or:

zPlainDate({ message: "error message" })

Maybe they could be both validator objects and a function that returns validator objects?

[macalinao]

I'm fine with a breaking change here, since we will make a breaking change to upgrade to zod v4.