Metafield completion

[charlespwd]

Is your feature request related to a problem? Please describe.
Accessing metafield values in a theme is kind of complicated.. There's probably something smart we can do that would make it super easy to have code completion for those in a shop-aware scenario.

Describe the solution you'd like

• Use dependency injection of some kind of data structure so that the browser and node environments have access to a shop's metafield config
• Wire the metafield definitions into the type system so that code completion works as though they were normal types.
• It should be opt-in
• It should be as easy as logging in to get the stuff we need. (think get approval and we do the graphql call for you)
• Maybe a CLI command. Maybe reuse CLI auth. Maybe dump to file (so that it's usable in CI).

Describe alternatives you've considered

• Using dependency injection so that the core does the graphql calls to get the stuff. I don't like that because it creates a network dependency and error handling can get weird. I'd like to be able to keep running theme-check in CI without making API calls. Probably something like dumping the definitions to a file and providing those. In web, we could probably do the graphql query to get them.

Additional context
Add any other context or screenshots about the feature request here.