Metafield theme-check

[aswamy]

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

Follows up #502
We need to verify if metafields being used in the code are valid. If the metafield does not exist, you should see a warning when running shopify theme check

Describe the solution you'd like

For themes:

• running shopify theme check should verify if the metafield exists on your shop
• If you don't have the metafield, you should get a warning message

For theme app extensions:

• TAEs might have their own metafield definitions that are specific to it (see https://shopify.dev/docs/apps/build/custom-data/metafields/use-app-data-metafields)
• running shopify theme check should verify if the metafield exists on your shop OR if the metafield is defined for the app itself
• If you don't have the metafield, you should get a warning message

Tasks

• Update theme-check in theme-tools to error out when metafield definitions dont exist
• Add lint ignore rules in Dawn + Horizon where we use metafields that don't exist in any shops by default
• Update CLI so there is a warning when running theme check and metafield isnt defined

NOTE: There are a few differences when you pull metafield definitions from CLI or from Theme Access App via CLI