`exporter_map` is deprecated, so let's use the list of exporters fetchedfrom the installed entrypoints.There's a supposed attribute `export_from_notebook` that should be setto a friendly string name if the exporter should be exposed in thefront-end. However, the exporters defined in `nbconvert` don't have itset, so I haven't used it to determine inclusion in the list. Instead,I've used the entrypoint name as the friendly name, which looks like itwas the intention from the way they are named.I ran the unit tests and tried starting up the notebook server andaccessing the API endpoint to verify the JSON looked correct.