Extend code assist on $ref values to propose objects from all JSON or YAML files

[tedepstein]

Extracted from #488. While editing an OpenAPI 2.0 (Swagger) or 3.0 file, extend the proposal list on $ref values to include valid objects from any JSON or YAML file in a designated scope, not just the reusable objects defined in OpenAPI files.

There are many design aspects to consider in a feature like this:

• Built-in scopes: document, project, workspace
• User-defined scopes:
  • path in the local filesystem
  • HTTP-accessible file collections (e.g. GitHub project, WebDAV)
• How to locate recognizable objects within a file
  • root object only
  • common locations, e.g. /definitions
  • user-configurable JSON Pointer or JMESPath
    • Global: JSON Pointer or JMESPath, applies to all JSON or YAML files
    • File type-specific: JSON Pointer or JMESPath, applies to JSON or YAML files of a specific subtype, with criteria to recognize files of that subtype.
• Scope cycling
  • Include with existing project and workspace scopes
  • Add new scopes
  • Make scope cycling user-configurable