Add bulk export option

[slonoed]

Currently, user can only export one prefab at once (export selected). This may be inconvenient when dealing with many files.
It would be helpful if menu could work on selected folder and then traverse it.

[hybridherbst]

The described functionality can already be done via the API.

Would be great to learn a bit more about what the goal is here – not having to use the API, but some kind of batch export being built-in? Not sure if selecting a folder and exporting that conveys enough about what is actually supposed to be exported (only toplevel files? only prefab variants? Which of those go into one file vs. being individual files?)