async function iconDownload( user ) {
	// don't download logo again if it has already been downloaded
	const { id, profile_image_url } = user;

	let file = userIconCache.get( id );
	if ( !file ) {
		file = await download( profile_image_url, iconCacheDir );
		userIconCache.set( id, file );
	}

	return file;
}
@param {TwitchUser} user
@returns {Promise<string>}