Index instance variable references

[vinistock]

The Ruby LSP provides find references and rename for instance variables. Indexing instance variables references is quite similar to constant references in the sense that we can resolve them during resolution with no other type checking steps.

Even the structure would be quite similar as I expect we would need to remember: uri_id, offsets and the name_id of that represents the self context for that variable.

Let's index instance variables and make sure that they are being resolved.