function split_headers(headers) {
	/** @type {Record<string, string>} */
	const h = {};

	/** @type {Record<string, string[]>} */
	const m = {};

	headers.forEach((value, key) => {
		if (key === 'set-cookie') {
			if (!m[key]) m[key] = [];
			m[key].push(...set_cookie_parser.splitCookiesString(value));
		} else {
			h[key] = value;
		}
	});

	return {
		headers: h,
		multiValueHeaders: m
	};
}
Splits headers into two categories: single value and multi value
@param {Headers} headers
@returns {{
  headers: Record<string, string>,
  multiValueHeaders: Record<string, string[]>
}}