Add transformer concept

[ItsHarper]

Validators are great since they allow the library to handle re-prompts, but using them can lead to duplicating work. If I need to parse a free-form answer and have the library handle re-prompt when the input is invalid, I have to have the validator run the parser, and see if it succeeds. If it does, I'm not given the result, so I have to re-run the parser.

It would be great to have a Transformer type that looks something like this:

interface Transformer<Input, Transformed> {
    transform: (input: Input) => TransformationResponse<Transformed>;
}

interface ValidTransformationResponse<T> {
    isValid: true;
    transformed: T;
}

type TransformationResponse<T> = InvalidResponse | ValidTransformationResponse<T>;

That way, I can perform validation and parsing all in one step, while still letting the library handle re-prompts.

Another thing that's sub-optimal about the duplicate-parsing approach is that when you're re-parsing, if the possibility of errors is embedded in the type of the return value of the parsing function, I have to write code to handle that possibility to satisfy the type system, when I already know that parsing will succeed if the parser is deterministic.

[ItsHarper]

This idea was definitely inspired by the Transforms concept in zod, btw