Allow recursive library paths

[heraplem]

Is your feature request related to a problem? Please describe.

I like to keep my library documents in subdirectories, one for each library entry. In order to make this work, I need to add each subdirectory to citar-library-paths. Additionally, whenever I add a new library file, I have to go and change citar-library-paths. It would be nice to instead have the possibility of letting citar-library-paths be interpreted recursively.

Describe the solution you'd like

Add a new variable, e.g., citar-library-paths-recursive, that, when true, makes Citar look in all subdirectories of each entry in citar-library-paths. This new variable would be set to nil by default for backwards compatibility.

Describe alternatives you've considered

The obvious alternative is to transpose the directory layout; i.e., have a separate directory for each file type. This won't work for my use-case, which is to have Xournal++ files linked to my PDFs.

The other solution is the null solution; i.e., just use a flat directory structure.