/**
 * Custom parsing for the Xdebug mode set via yargs. This function ensures three things:
 * 1. If the --xdebug flag was not set, set it to 'off'.
 * 2. If the --xdebug flag was set by itself, default to 'debug'.
 * 3. If the --xdebug flag includes modes, make sure they are accepted by Xdebug.
 *
 * @param {string|undefined} value The user-set mode of Xdebug; undefined if there is no --xdebug flag.
 * @return {string} The Xdebug mode to use with defaults applied.
 */
export default function parseXdebugMode(value: string | undefined): string;