Add JsonMissingBlock Check

[navdeep5]

What are you adding in this PR?

Addresses https://github.com/Shopify/shopify/issues/552694

This PR introduces enhanced validation checks for JSON template files, specifically ensuring that:

1. Block types exist as valid files in the blocks/ dir
2. Block types are allowed in their respective section or block at the root-level

What's next? Any followup issues?

With this being a new check, we will want to update https://github.com/Shopify/shopify-dev so users can understand their errors. Developer Docs changes: https://github.com/Shopify/shopify-dev/pull/51342

What did you learn?

I have never worked or come across JSON template files before so this was fun to learn about.

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • It's in the allChecks array in src/checks/index.ts
  • I ran yarn build and committed the updated configuration files
    • If applicable, I've updated the theme-app-extension.yml config

• I included a minor bump changeset
• My feature is backward compatible

• I included a patch bump changeset

[miazbikowski]

I noticed that if I just open my json file and put whatever non-existent type as a block type, it doesn't complain. It will only complain if that non-existent block type is also in the parent's schema as an accepted block. Is that by design? I was wonder if we should verify the json block types for existence independently of the schema, but I figured maybe there's a reason for this behaviour.

[miazbikowski]

We have that issue with onCodePathEnd checks not running when the editor opens, but rather on refocusing on the file tab, but that's not strictly related to this check. Otherwise, I tophatted and code lgtm.

[miazbikowski]

Bit of a captain obvious but I'll write it out just in case, but I didn't check that the util functions don't duplicate logic that's also used elsewhere and that we'd benefit from extracting out of that check. I'm thinking you just kept that's specific to that check in there.

[albchu]

Change LGTM and tophats as expected! Great work Nav! I added one minor nit but feel free to ingore 😏