rbs prototype rbi should convert T::Enumerator properly

In Sorbet, T::Enumerator only has one argument (e.g. T::Enumerator[String]), but in ruby-signature it has 2.

Right now, this is the input

class Foo
  sig { returns(T::Enumerator[String]) }
  def bar; end
end

And this is what rbs prototype rbi outputs:

class Foo
  def bar: () -> Enumerator[String]
end

The rbs above causes this error when parsed:

::Enumerator expects parameters [Elem, Return], but given args [::String] (Ruby::Signature::InvalidTypeApplicationError)

I'm not 100% sure, but I think T::Enumerator[String] should be converted to Enumerator[String, self] in ruby-signature?

Start with the rbs prototype rbi entry point and reproduce the conversion using the T::Enumerator[String] example from the issue. Trace the RBI-to-RBS type conversion and determine the correct two-parameter Enumerator representation. Done means the generated RBS parses successfully and expresses the intended enumerator type.