tryScanUnicode() {
    const offset = this.stream.pos();
    if (!this.stream.eos() && this._unicodeRange()) {
      return this.finishToken(offset, TokenType.UnicodeRange);
    }
    this.stream.goBackTo(offset);
    return void 0;
  }
Read the range as described in https://www.w3.org/TR/CSS21/syndata.html#tokenization
Assume the `u` has aleady been consumed
@returns if reading the unicode was successful