Feature: Only export used types, don't export unused types

[hybridherbst]

Currently, each JSON export always exports all types.

This might lead to some viewers implementing the extension to incorrectly assume that all types always need to be present and have a specified order, which I believe is not the case.

I think it would be great if only the required types for a graph are exported (so if all nodes operate on "float", then only the float type would be exported).