(feat) Add optional `transform` callback

[justsml]

Path 1: Simplify OptionTypeConfig

Where transform becomes THE WAY to do validation, defaults, and the expected return type, etc.

export type OptionTypeConfig =
  | OptionTypeEnum
  | {
      transform?: (input: unknown) => OptionType;
    }
  | {...} //

Path 2: Add transform to all existing type's

This would run through all existing validation and apply defaults, then run it through any specified transform().

| Seems trickier with TS.