Make script return value converters more ergonomic #61389
Description
Right now, script return value converters are attached to the context, and require handling all boxed casts in the convertFromDef converter.
The converters as-is fully implement the requirements. We can make it easier for implementer by potentially pulling them out as a class and potentially call convertFromX with boxed def types rather than requiring implementers handle them.
Related: #59647