Open Zod to externally-defined combinators

[carlpaten]

To define a ZodTypeDef instance, one must pick a member of the enum ZodTypes. This is a problem for users wishing to define their own instances of ZodType, which requires a ZodTypeDef instance.

The solution is to factor the centralized aspects of Zod such that every combinator owns its error reporting, parsing logic, etc.

[carlpaten]

@colinhacks I'm not expecting you to implement this, but if you're open perhaps we could determine a process that would make you feel comfortable accepting PRs going in this direction.

[colinhacks]

Just released zod@3.0.0-alpha.7 which eliminates ZodTypes. Now any subclass of ZodType that properly implements the _parse method is a first-class citizen.