function readUnicodeSequence(reader) {
	const regExp = /\{(?<hexDigits>[\dA-Fa-f]+)\}/uy;

	regExp.lastIndex = reader.pos;
	const match = regExp.exec(reader.source);

	if (match) {
		const codePoint = parseInt(match.groups.hexDigits, 16);

		reader.pos = regExp.lastIndex;
		return String.fromCodePoint(codePoint);
	}
	return readHexSequence(reader, 4);
}
Reads a Unicode escape sequence.
@param {TextReader} reader The reader should be positioned after the "u".
@returns {string} A code unit.