Bugfix bibitem undefined

[nolanking90]

Fixes #1171
Add a bibitem list to the latex Semantics struct, bibitem to CommandName and SyntaxKind enums, and handlers for processing bibitem nodes in a tex document.
Added checks against the bibitem list for undefined references and unused references.

Tested in NVIM v0.11.0-dev-386+g545aafbeb
w/ following minimum working example

\documentclass{article}

\begin{document}

See~\cite{ref1} % No error
See~\cite{ref2} % Undefied Reference Error

\begin{thebibliography}{9}
  \bibitem{ref1}
  Donald E. Knuth (1986) \emph{The \TeX{} Book}, Addison-Wesley Professional.

  \bibitem{ref3} % Unused Reference Warning
  An unused reference
\end{thebibliography}

\end{document}