/**
 * Extends an object with another one combining them whenever it's possible.
 * If both objects (or it's children) have an array on the same property those will be concatenated.
 * If both objects (or it's children) have an object on the same property those will be merged.
 * Otherwise it will be overwriten.
 *
 * @param {Object} target Object to be modified.
 * @param {Object} source Object where properties will be copied from.
 * @returns {Object} The object passed as the first argument.
 */
export default function deepExtend<T extends object, U extends object>(target: T, source: U): T & U;