Add top left table cell separation like `\` to precise titles of first cells of lines and columns

[Benjamin-Loison]

\begin{tabular}{ | c | c | c | c | }
    \hline
    Configuration $\backslash$ Metric & Precision & Recall & F1 score\\
    \hline
    Article complete (with dropout and attention) & 0.8082 & 0.4602 & 0.5432\\
    \hline
    Reproduced complete with their embeddings & 0.7165 & 0.7909 & 0.6497\\
    \hline
    Reproduced incomplete (without dropout and attention) with their embeddings & 0.8946 & 0.9055 & 0.6867\\
    \hline
\end{tabular}

Brief outline of the enhancement

LaTeX2e generally cannot add new features without an extreme amount of care to accommodate backwards compatibility. Please do not be offended if your request is closed for being infeasible.

Minimal example showing the current behaviour

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}

  % Any preamble code goes here

\begin{document}

  % Demonstration of issue here

\end{document}

Minimal example showing the desired new behaviour

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}

  % Any preamble code goes here

\begin{document}

  % Demonstration of issue here

\end{document}