Add additional diagnostics

[pfoerster]

• Report a hint if
  • a label is unused
  • a label reference is undefined
  • a citation is undefined
  • a BibTeX entry is unused
• Extract the diagnostics module to a separate crate

[clason]

Nice! Some comments:

1. Undefined references/citations should be an actual error (or at least a warning).
2. Should only the unused label itself or the full \label command be tagged? I'd say the former, for symmetry with how undefined labels and unused bibtex keys are handled. (Also makes it easier to quickly distinguish unused labels and commented out label commands.)
3. Unused labels/entries can spam the diagnostic list quite a bit; but I guess as these are the only hints, you can just filter them out.
4. I have to admit I'm a bit on the fence about tagging unused bibtex keys; many people use their bibtex file as a sort of database, and I admit it looks a bit weird that such a structured file has keys sometimes colored, sometimes grayed out. On the other hand, it makes it much easier to clean up a bib file for submission, so having that diagnostic is certainly helpful.

[pfoerster]

Undefined references/citations should be an actual error (or at least a warning).

Yeah, I think, texlab can be more confident here and report an error like we do for syntax errors.

I'd say the former, for symmetry with how undefined labels and unused bibtex keys are handled.

I agree, graying out the entire BibTeX entry does not look good and is very distracting (especially when using a very large file with many unused entries).

[clason]

Sorry for the necro-comment, but wanted to note this down shortly before I forget: Another additional diagnostic that could be very useful is a "duplicate label/bibtex entry" (probably a Warning?)