ebib: add support for multiple entries using vertico

[atanasj]

Not sure if this is currently possible, but adding support for this with vertico would be great!

See: https://github.com/emacs-citar/citar/pull/163 for something similar.

[joostkremers]

Hi, have you tried setting ebib-citation-insert-multiple to t? I'm using selectrum at the moment, though considering trying out vertico. Since vertico is closely integrated into the default completion mechanism, from what I understand, setting this option may already get you a good part of the way there.