Add code action to add attribute for instance variable

[vinistock]

Add new code actions to add a new attr_reader, attr_writer or attr_accessor for the given instance variable.

# BEFORE
class Foo
  def initialize
    @name = "Bar"
  end
end

# AFTER
class Foo
  attr_reader :name
 
  def initialize
    @name = "Bar"
  end
end

Here's an example PR adding a refactor end to end #2372.