function readOctalSequence(reader, maxLength) {
	const [octalStr] = reader.read(-1, maxLength).match(/^[0-7]+/u);

	reader.advance(octalStr.length - 1);
	const octal = parseInt(octalStr, 8);

	return String.fromCharCode(octal);
}
Reads an octal escape sequence.
@param {TextReader} reader The reader should be positioned after the first octal digit.
@param {number} maxLength The maximum number of octal digits.
@returns {string} A code unit.