Row group labels as a column in stub

[rich-iannone]

This PR brings us the ability to display row group labels as a column in the stub. This is made possible with the new option row_group.as_column in tab_options() (default keeps the preexisting behavior).

Here is an example:

library(gt)

exibble %>%
  gt(
    rowname_col = "row",
    groupname_col = "group"
  ) %>%
  tab_header(
    title = md("Data listing from **exibble**"),
    subtitle = md("`exibble` is an R dataset")
  ) %>%
  fmt_number(columns = num) %>%
  fmt_currency(columns = currency) %>%
  tab_footnote(
    footnote = "Using commas for separators.",
    locations = cells_body(
      columns = num,
      rows = num > 1000
    )
  ) %>%
  tab_footnote(
    footnote = "Using commas for separators.",
    locations = cells_body(
      columns = currency,
      rows = currency > 1000
    )
  ) %>%
  tab_footnote(
    footnote = "Alphabetical fruit.",
    locations = cells_column_labels(columns = char)
  ) %>%
  tab_style(
    style = cell_fill(color = "lightblue"),
    locations = cells_row_groups(groups = "grp_a")
  ) %>%
  tab_options(row_group.as_column = TRUE) %>%
  summary_rows(
    groups = TRUE,
    columns = c(num, currency),
    fns = list(
      min = ~min(., na.rm = TRUE),
      max = ~max(., na.rm = TRUE),
      avg = ~mean(., na.rm = TRUE)),
    formatter = fmt_number,
    use_seps = FALSE
  )

And this is how it appears:

Fixes: #291

[jcheng5]

This is a redundant if

[jcheng5]

Simplify create_group_dividers_l