No way to add stop strings with `drama_llama` backend

This isn't enabled because the egui text entry boxes escape certain sequences and so it's not possible to add, for example `\n` or `\t`. Currently there's a button for the former.

If we allow adding arbitrary stop strings it will lead to unexpected results. It may be necessary to write or find a raw text edit widget. Or maybe there is some functionality in `egui` that I am missing.