LSP: Support instance variable definition jump

[kateinoigakukun]

Description

Added support of Go to instance variable definition.

Known Issue

In get_foo, @foo can't find the its definition in set_foo due to the limitation of current identify_class_for_ivar analysis

class A
  def set_foo
    @foo = 1
  end
end

class B < A
  def get_foo
    @foo
  end
end

Demo

Screen.Recording.2021-08-10.at.16.58.19.mov