Fix LatexBibliographyReferenceProvider to provide autocompletion also…

[fberlakovich]

… after the first cite entry

Fix #1564

Summary of additions and changes

• Enables the LatexBibliographyReferenceProvider to provide completions for more than the first entry

How to test this pull request

\documentclass[11pt]{article}
\usepackage{natbib}

% Document
\begin{document}

    \cite{FirstKey2020,<Ctrl + Space>}
    \bibliography{bibtex}
    \bibliographystyle{plain}

\end{document}```