Fix instance variable object encoding with extended type information

[madsmtm]

Instance variables include extended type information, we should find some way to represent this.

See: https://bou.io/ExtendedTypeInfoInObjC.html

[madsmtm]

While fairly easy to parse, I'm yet unsure of how it should be represented in objc2-encode, since we want objc2::encode::Encode::ENCODING to return something that can always be used, regardless of context. So e.g. if we have something with Encoding::Object(None), how should that compare to Encoding::Object(Some("MyClass"))? And what should the Display implementation default to outputting?

[madsmtm]

I'm going to just parse and ignore it for now