[GDExtension] Allow having generic classes as array-like in gdscript

In GDScript, when you make custom classes with gdext generally this is the way:

`var myExtClass:CustomClass = CustomClass.new()`

but what if I want to have an array-like class with generics? its currently possible? Something like:

`var test:CustomClass[float] = []`

this means that in cpp the customclass is:

```cpp
class CustomClass : public RefCounted {...};
```

where T is really T or it could be Variant or Object to support other custom classes