Pandoc-citeproc is no longer maintained. Pandoc now uses the citeproc
library, and no external filter is needed. Also use nixpkgs-fmt to
format archive.
Some files in the appimage changed from 'zettlr' to 'Zettlr', keeping
them as lowercase in package output for consistency.
Add texlive and pandoc{,-citeproc} for PDF export.