code: function getCommentSL() {
    const type = NodeType.CommentSLType;
    const token = tokens[pos];
    const line = token.ln;
    const column = token.col;
    const content = tokens[pos++].value.substring(2);
    const end = !content ? [line, column + 1]
      : getLastPosition(content, line, column + 2);
  
    return newNode(type, content, line, column, end);
  }
docs: Get node with a single-line comment.
  @return {Array} `['commentSL', x]` where `x` is comment's message
  (without `//`)