Pass `self_receiver` to Rubydex for completion in class/module bodies

[Morriar]

Context

Follow-up to Shopify/rubydex#794 which makes self type required for completion.

After that PR lands, the Ruby LSP needs to pass the self_receiver when requesting completions so that completion works correctly inside class/module bodies.

Completion is currently incorrect in class/module bodies for constants, class variables, methods, and instance variables.

Tasks

• Update the Ruby LSP to pass self_receiver to Rubydex completion requests
• Add completion tests verifying expected behaviour inside class/module bodies for:
  • Constants
  • Class variables
  • Methods
  • Instance variables

References

• Make self type required for completion rubydex#794