Validate string attrs have opening and closing quotes

[nvanexan]

Closes #114

[mfix-stripe]

Sweet! This looks good to me, but want to pass it to @rpaul-stripe for a quick review.

[rpaul-stripe]

Hey, I'm sorry it's taken so long to respond to this PR. Your change to the validator raised some false positives on sample content and I had to set aside some time to dig into it in order to understand why.

I think the main problem is that it's checking every string-type attribute value instead of just the content attribute of raw text nodes. This results in false positives in several places, namely any fenced code block that contains a tag.

The real underlying problem here is with the way that Markdoc identifies the end of a tag construct in content. When it sees a {% in the text, it scans forward each character until it finds a matching %} that is not inside of a string. It does this in order to avoid prematurely identifying the tag end when it encounters tag-like syntax inside of a string attribute value. This means that if a tag has an unterminated string, the parser assumes that the actual closing is still inside of that string and it declines to match it as a tag.

What we should probably do is solve this in the markdown-it plugin and make the parser generate an error for the whole block any time it sees the tag start syntax without a matched end. I think this would be cleaner than trying to identify malformed tags during the validation process. I will tackle fixing this myself at some point in the next week or two.

For now, if you want to be able to catch these cases reliably during validation without a large surface area of potential false positives, I would suggest overriding the text schema node and doing the validation check there. Here's a quick-and-dirty example of something you can include in your Markdoc config to achieve this:

export const text: Schema = {
  attributes: {
    content: { type: String, required: true },
  },
  transform(node) {
    return node.attributes.content;
  },
  validate(node: Node, config: Config) {
    if (node.attributes?.content?.match?.(/{%[^%}]+%}/)) {
      return [{
        id: 'attribute-value-invalid',
        level: 'error',
        message:
          'The string attribute must have an opening and closing quote',
      }]

    return [];
  }

Thanks for raising this issue!