Ensure that the row group labels in a two-column stub can render footnote marks

[rich-iannone]

When formatting a table with a two-column stub arrangement, the group label wasn't being used to generate the cell matrix (the left-most cell corresponding to a group label was taken from the table body). This is wrong as the group labels are placed in the groups_rows_df table, where they undergo rendering and that's where footnote marks are affixed to them.

This PR modifies the body row rendering in the separate HTML, LaTeX, and RTF implementations. Several snapshot tests have been added to capture the fix. Here is the code that illustrates the problem, with a screenshot showing that the change was successful.

pizzaplace %>%
  dplyr::filter(name %in% c("soppressata", "peppr_salami")) %>%
  dplyr::group_by(name, size) %>%
  dplyr::summarize(`Pizzas Sold` = dplyr::n(), .groups = "drop") %>%
  gt(rowname_col = "size", groupname_col = "name") %>%
  summary_rows(
    groups = TRUE,
    columns = `Pizzas Sold`,
    fns = list(TOTAL = "sum"),
    formatter = fmt_number,
    decimals = 0,
    use_seps = TRUE
  ) %>%
  tab_options(row_group.as_column = TRUE) %>%
  tab_footnote(
    footnote = "The Pepper-Salami.",
    cells_row_groups(groups = "peppr_salami")
  )

Fixes: #1001

[cscheid]

See if it makes sense to change the line on utils_render_html.R as I suggested, otherwise LGTM.

[cscheid]

lgtm!