Rename bibtex package bibliography and make loaders modular

[alerque]

I've the feeling that the bibtex package, providing \cite, \reference, \loadbibliography etc. is somewhat ill-named, it should be bibliography, and support for bibliography formats should be provided in their own files. Besides .bib, one could want to provide references in RIS, Refer, EndNotes... or even CSL-JSON, eventually...). It's a bit soon to say, maybe, and I didn't want to split things too early here.

Whatever, having the LSP instructed for type safety and parameter meanings (regardless of an eventual doc generation) is always nice, no?

Originally posted by @Omikhleia in #2048 (comment)

Yes I agree this should be a bibliography package with somewhat modular support for loaders. We can probably go ahead and rename the package and just provide the old name as a shim for now so it doesn't have to be a breaking change yet, just a deprecation notice on the old name.

As for the docstring hints, sure, and it should affect editors editing that file. Just be aware it won't get published since it isn't a public function.

Originally posted by @alerque in #2048 (comment)