Warn against missing use of `{{ block.shopify_attributes }}` in blocks with `tag: null`

[charlespwd]

Is your feature request related to a problem? Please describe.

“When ”tag”: null set in block schema, then {{ block.shopify_attributes }} must exist in the markup of the template”

Describe the solution you'd like
For block/*.liquid files, when the {% schema %} contains "tag": null at the top level, then we should report a WARNING severity offence on tag: null

Additional context

https://shopify.dev/docs/storefronts/themes/architecture/blocks/theme-blocks/schema#tag:~:text=of%20the%20block.-,Caution,index%20when%20merchants%20re%2Dorder%20blocks%20without%20leaving%20orphaned%20HTML%20elements.,-Copy

[benjaminsehl]

Regex to match tag against obviously should be something like \{\{-?\s?block\.shopify_attributes\s?-?\}\}

[charlespwd]

Ah we can do that with AST visiting.

Should be something like

let found = false;
let tagNullNode = null;

return {
  LiquidVariable(node, ancestors) {
    if (node.name !== 'block') return
    if (!node.lookups.find(lookup => lookup.value === 'shopify_attributes')) return;
    found = true;
  }

  LiquidRawTag(node) {
   if (node.name !== 'schema') return;
   // ... make sure it has tag: null
  }

  onCodePathEnd() {
    if (!found && tagNullNode) {
      context.report(/* ... */);
    }
  }

[benjaminsehl]

From @mmorissette on Slack:

When tag: null is set in the block's schema, the block's top wrapping element must include {{ block.shopify_attributes }} for it to be recognized by the theme editor. That is how we tell what consists of a block in the preview.

This PR that Ben shared earlier is the simple use case where the block's wrapping div element didn't contain {{ block.shopify_attributes }}

As Nathan mentioned, there are more complex examples where the content of the block comes from a snippet (i.e. snippets/group.liquid) in which case theme check would technically need to check the rendered snippet to ensure that {{ block.shopify_attributes }} is present.

I've also seen examples where the block attributes was accidentally rendered multiple times (slack thread)

I think we could address this in phases

1. Simple use case where theme-check only looks at the current file (which should address a majority of the cases)
2. Checking against rendered snippets
3. Checking to see if {{ block.shopify_attributes }} is accidentally rendered multiple times