Support a way to transform (map) a value before validating it

[nesk]

It could be useful to transform a value before validating it, for example:

Validator<String> {
    this.map { it.trim('.') }.isNotBlank()
}

By "transforming" the value, the path remains the same, until overridden by withPath.

Essentially, it would work like Optional.map() in Java, or Either.map() with Arrow, but instead of returning an Optional or an Either, it returns a Validatable.