Add modify_split_caption() to label paginated splits and hide the split column

[Melkiades]

When paginating a listing with gtsummary::tbl_split_by_rows(variable_level = <col>) (e.g. splitting a lab listing by PARAM, or a demographics listing by treatment), two manual post-processing steps are always needed:

1. The split value is stored in attr(tbl, "variable_level") as a bare level (e.g. "WEIGHT"). Decorators (citril) inject it as the page subtitle, but it usually needs a prefix, e.g. "Parameter: WEIGHT".
2. The split column is redundant on each single-value page and should be hidden.

Currently this is repeated inline in every paginated template:

gts |>
  purrr::map(\(tbl) {
    lvl <- attr(tbl, "variable_level")
    tbl <- gtsummary::modify_column_hide(tbl, columns = "PARAM")
    attr(tbl, "variable_level") <- paste0("Parameter: ", lvl)
    tbl
  }) |>
  structure(class = class(gts))

Proposal

Add modify_split_caption() to R/tbl_listing.R (sibling of remove_duplicate_keys() / add_blank_rows(), shared @rdname tbl_listing):

modify_split_caption(
  x,
  spl_col,
  pattern = "Parameter: {spl_level}",  # glue; {spl_level} is the split value
  hide_spl_col = TRUE                   # hidden silently; no-op if col absent/already hidden
)

• Rewrites each split page's variable_level label via the glue pattern (default suits BDS/lab tables split by PARAM).
• Hides spl_col on every page by default; silent when the column is absent or already hidden.
• Works on a single tbl_split element or the whole list.
• No citril/internal references — operates only on the gtsummary-native variable_level attribute, so crane stays open-source clean.