Create a type registry

Link type name with type object:

Integer -> VBAInteger

When a project defines a UDT or a new object, add it to the registry.
Currently, the interpreter saves a copy of the object when a function specifies a return type, but if it specifies a class, it doesn't know what to add. The interpreter should instead just save the name of the return type, then look up the type in the registry when the function is called.