function _merger(key, target, source, options) {
    if (!isValidKey(key)) {
        return;
    }
    const tval = target[key];
    const sval = source[key];
    if (isObject(tval) && isObject(sval)) {
        // eslint-disable-next-line @typescript-eslint/no-use-before-define
        merge(tval, sval, options);
    } else {
        target[key] = clone(sval);
    }
}
The default merger when Chart.helpers.merge is called without merger option.
Note(SB): also used by mergeConfig and mergeScaleConfig as fallback.
@private