/**
 * Masks a string, leaving only the last N characters visible
 *
 * @param value - The string to mask
 * @param visibleChars - Number of characters to leave visible at the end (default: 4)
 * @param maskChar - Character to use for masking (default: '*')
 * @param preservePrefix - Number of characters to preserve at the start (optional)
 *
 * @example
 * maskString('+37369313123', 4)           // '+373*****3123'
 * maskString('+37369313123', 4, '*', 4)   // '+373*****3123'
 * maskString('user@example.com', 4)       // '***********.com'
 * maskString('1234567890', 4)             // '******7890'
 */
export declare function maskString(value: string, visibleChars?: number, maskChar?: string, preservePrefix?: number): string;