/**
 * Returns a string formatted in its plural form.
 *
 * Handles common English pluralization rules, including:
 * - y → ies
 * - s/x/z/ch/sh → es
 * - known irregulars (e.g., child → children)
 * - acronym exceptions (e.g., API → APIs)
 *
 * The output preserves the casing of the input, especially the prefix.
 *
 * @example
 * ```ts
 * toPlural("city"); // "cities"
 * ```
 *
 * @example
 * ```ts
 * toPlural("box"); // "boxes"
 * ```
 *
 * @example
 * ```ts
 * toPlural("child"); // "children"
 * ```
 *
 * @example
 * ```ts
 * toPlural("person"); // "people"
 * ```
 *
 * @example
 * ```ts
 * toPlural("API"); // "APIs"
 * ```
 *
 * @example
 * ```ts
 * toPlural("ID"); // "IDs"
 * ```
 *
 * @example
 * ```ts
 * toPlural("File"); // "Files"
 * ```
 *
 * @example
 * ```ts
 * toPlural("buzz"); // "buzzes"
 * ```
 *
 * @example
 * ```ts
 * toPlural("CPU"); // "CPUs"
 * ```
 *
 * @example
 * ```ts
 * toPlural("DOG"); // "DOGS"
 * ```
 *
 * @example
 * ```ts
 * toPlural("A"); // "As"
 * ```
 *
 * @param word A singular word to be pluralized.
 * @returns The plural form of the input word.
 */
export declare function toPlural(word: string): string;