fix: ensure that cells_stub() works with LaTeX output tables

[rich-iannone]

This PR fixes the use of cells_stub() for LaTeX output tables. We can now properly target all stub columns when columns = NULL for multi-column stubs. This ensures that styles and footnotes are applied to every stub column rather than just the first one.

This gt code:

exibble[, c(1, 2, 8, 9)] |>
  gt(
    rowname_col = c("group", "char")
  )|>
  tab_style(
    style = list(
      cell_text(align = "right")
    ),
    locations = list(cells_stub()) 
  ) |>
  as_latex() |>
  cat()

Now produces this LaTeX output:

\begin{table}[t]
\fontsize{12.0pt}{14.0pt}\selectfont
\begin{tabular*}{\linewidth}{@{\extracolsep{\fill}}l|l|rl}
\toprule
\multicolumn{2}{c}{} & num & row \\ 
\midrule\addlinespace[2.5pt]
\multicolumn{1}{r}{{grp\_a}} & \multicolumn{1}{r}{{apricot}} & 1.111e-01 & row\_1 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{banana}} & 2.222e+00 & row\_2 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{coconut}} & 3.333e+01 & row\_3 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{durian}} & 4.444e+02 & row\_4 \\ 
\multicolumn{1}{r}{{grp\_b}} & \multicolumn{1}{r}{{NA}} & 5.550e+03 & row\_5 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{fig}} & NA & row\_6 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{grapefruit}} & 7.770e+05 & row\_7 \\ 
\multicolumn{1}{r}{{}} & \multicolumn{1}{r}{{honeydew}} & 8.880e+06 & row\_8 \\ 
\bottomrule
\end{tabular*}
\end{table}

Notice the use of the \multicolumn{1}{r}{{...}} in the first two columns (this is a two-column stub).

Fixes: #2103

[thebioengineer]

I just opened #2105 🤣

[rich-iannone]

@thebioengineer you are awesome! Yeah, the gt package is getting a lot of (overdue) attention today :)

[rich-iannone]

I don't think I've ever had a situation like this but @thebioengineer , could you close #2105? This PR is now merged. I'll definitely review your #2101 PR though!