/**
 * Generates a unique token.
 * @param {number} length - The desired length of the token. Must be at least 8. Default: 20.
 * @param {'alphanumeric' | 'hexadecimal'} mode - The mode of the token, either 'alphanumeric' or 'hexadecimal'. Default: 'alphanumeric'.
 * @returns {string} - A unique token of the specified length.
 * @throws {Error} - Throws an error if the length is less than 8.
 * @note - The function generates a token based on a UUID and ensures uniqueness. A cache is used to avoid collisions.
 */
export declare function generateUniqueToken(length?: number, mode?: 'alphanumeric' | 'hexadecimal'): string;