Added missing schema check for theme app extension

[AribaRajput]

What are you adding in this PR?

cc (collab): @itsjustriley

Closes #402.

Shopify dev docs PR: https://github.com/Shopify/shopify-dev/pull/50817

Adds a theme check on theme app extensions for missing schema.

What's next? Any followup issues?

No followup issues.

What did you learn?

Cannot pass context into codePathStart() or codePathEnd(). Theme app extensions cannot currently be tophatted in Horizon.

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

[charlespwd]

🤩 LGTM! Let's go!

[AribaRajput]

Used code from: https://www.shopify.com/ca/partners/blog/theme-blocks

[AribaRajput]

/shipit