Base class for custom types.

Custom type example:

import { Type, ArgumentValue } from "./mod.ts";

export class ColorType extends Type<string> {
  public parse({ label, name, value, type }: ArgumentValue): string {
    if (["red", "blue"].includes(value)) {
      throw new Error(
        `${label} "${name}" must be of type "${type}", but got "${value}".` +
        "Valid colors are: red, blue"
      );
    }
    return value;
  }

  public complete(): string[] {
    return ["red", "blue"];
  }
}