/**
 * Returns the length of HTML entity that is a prefix of
 * the given string (excluding the '&'), 0 if it
 * does not start with an entity.
 *
 * @example
 *   entityPrefixLength('&copycat') // 4
 *   entityPrefixLength('&foo &amp &bar') // 0
 *
 * @param {string} value - Input string.
 * @return {number} - Length of an entity.
 */
export default function entityPrefixLength(value: string): number;