Migrate find references to Rubydex

[vinistock]

Motivation

Migrate find references to use Rubydex. There are two things missing, which I encoded in the tests:

1. Indexing instance variable references (I have a branch going)
2. Exposing method reference receivers, so that we can be more precise about references

Implementation

I migrated the request in a way that it's already prepared to account for instance, class and global variables as soon as we add support for it in Rubydex.

Automated Tests

Added a bunch of new tests trying to encode all sorts of behaviours that the previous ones didn't. For example:

• Ensuring that the request only shows references if you have the cursor exactly on top of names
• The placeholder behaviour for global, class and instance variables
• Different variations of method references