code: function getString() {
    const type = NodeType.StringType;
    const token = tokens[pos];
    const line = token.ln;
    const column = token.col;
    const content = token.value;
  
    pos++;
  
    return newNode(type, content, line, column);
  }
docs: Get string's node
  @return {Array} `['string', x]` where `x` is a string (including
  quotes).