Support JSON schema validation

[dbarnett]

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.

[nogweii]

It would also be nice to support a configuration command to be able to have the editor manually specify the schema in case there isn't a $schema field already.