feat: add check citations command to see unused cites

[buehler]

Hi there :-)

I really like using your extension and thought to add a new command for BibTeX files. It would be very cool to have this inserted into the workshop extension.

This PR adds a new command to the action-bar under the bibtex category. It allows people to use the "checkcites" command line tool that is shipped with recent Tex installations. The user then sees a list of unused citations and can directly jump to them.

As such, you can cleanup your citations at the end of your paper.

I know that I still need to provide translations. Do I need to do them for all languages? Because I'll not be able to do that.

I'll open this PR as a draft until I know how to proceed.