[TypeScript Codegen] Remove undefined type from optional types

[Adrian Zimmer]

Currently, types marked as optional (using the ?: notation) still contain the undefined type. This does not cause any issues but is also redundant.

To fix this, the type_loader function has to be modified in a way that the undefined type is not added to the instance_type field while still retaining information about the type being optional so that functions like declare_field can add the ?: notation.