function getFile(url, cb) {
	const id = Number.parseInt(Date.now() + Math.random() * 1000000);
	resolvers[id] = cb;
	self.postMessage({
		action: "get-file",
		data: url,
		id,
	});
}
Retrieves the contents of a file from the main thread.
@param {string} url
@param {function} cb