getFileTimestamp(path, callback) {
		const cache = this._fileTimestamps.get(path);
		if (cache !== undefined) return callback(null, cache);
		this.fileTimestampQueue.add(path, callback);
	}
@param {string} path file path
@param {(err?: WebpackError | null, fileTimestamp?: FileTimestamp) => void} callback callback function
@returns {void}