function readHexSequence(reader, length) {
	const str = reader.read(0, length);
	const charCode = parseInt(str, 16);

	reader.advance(length);
	return String.fromCharCode(charCode);
}
Reads a hex escape sequence.
@param {TextReader} reader The reader should be positioned on the first hexadecimal digit.
@param {number} length The number of hexadecimal digits.
@returns {string} A code unit.