Set Style/AccessorGrouping to separated

[JamieMagee]

While we've been doing the Sorbet migration, we've had to separate out all of our attribute accessors so we can add sigs to them. For example:

class Foo
  attr_reader :bar, :bax, :baz
end

has become

class Foo
  attr_reader :bar
  attr_reader :bax
  attr_reader :baz
end

This change enables the RuboCop Style/AccessorGrouping rule and sets it to separated so we don't have to do this change manually.

Check out the RuboCop documentation for more information.