Add definition, hover and completion support for class variables

[vinistock]

This issue is exactly the same as #2644, but for class variables. Please refer to the description of #2644 for instructions on how to implement it.

The work is exactly the same, but instead of handling globals $something, we handle class variables @@something.

Note: class instance variables are already handled. No work is needed on that front. That is:

class Foo
  @class_instance_variable = "Hello!"
end

[rogancodes]

Hi Team, I am looking at this issue.