/**
 * Ensures that a given URL string ends with a trailing slash.
 *
 * - If the URL already ends with a slash, it is returned unchanged.
 * - If not, a slash is appended at the end.
 *
 * @param url - The URL string to normalize.
 * @returns The URL string guaranteed to end with a trailing slash.
 *
 * @example
 * ```ts
 * addTrailingSlash('/docs');
 * // '/docs/'
 * ```
 */
export declare function addTrailingSlash(url: string): string;
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