Regal's parser should be made schema-aware

[anderseknert]

Having schemas to define things like the input document is a great way to prevent typos and mistakes that would otherwise only be caught at runtime. The opa check command allows providing a --schema directory, but there's no corresponding option for Regal.

Screenshot pictures a type error from typing an invalid input reference. This is caught by the "check on save" feature, but not by Regal so the error isn't inlined.

We should allow pointing out a schema directory via .regal/config.yaml and provide that to the parse if found.

[anderseknert]

Tagging this "language server protocol" but this should benefit linting from the CLI as well.

[anderseknert]

This could also be used for code completions when provided, so that typing inp (or input., whatever) would bring up suggestions based on the possible attributes from the schema.