Support JSON schema validation

Would it ever be possible for jsonnet-language-server to detect JSON schemas and use them to empower completion and warnings?

For instance, at the simplest extreme that could mean that if you have a simple jsonnet file with a "$schema" field, code completion could suggest other valid names for top-level JSON field names from the schema for you, and warn if you use a name that's not defined in the schema.

No files, tests, or entry points are named. Review the language server's existing completion and warning behavior, then clarify how $schema and JSON Schema support should be discovered and scoped; done should include agreed completion suggestions and validation warnings.

Written by the indexing model from the issue text.