combineSizes = (a, b, combine) => {
	const aKeys = new Set(Object.keys(a));
	const bKeys = new Set(Object.keys(b));
	/** @type {SplitChunksSizes} */
	const result = {};
	for (const key of aKeys) {
		result[key] = bKeys.has(key) ? combine(a[key], b[key]) : a[key];
	}
	for (const key of bKeys) {
		if (!aKeys.has(key)) {
			result[key] = b[key];
		}
	}
	return result;
}
@param {SplitChunksSizes} a first sizes
@param {SplitChunksSizes} b second sizes
@param {CombineSizeFunction} combine a function to combine sizes
@returns {SplitChunksSizes} the combine sizes