Add the `tab_row_groups_when()` function

[jcheng5]

Ability to specify multiple row groups in a dplyr::case_when fashion

exibble %>%
  gt() %>%
  tab_row_groups_when(
    group == "grp_a" ~ "A",
    group == "grp_b" ~ c("Group B" = "B"),  # label is "Group B", group_id is "B"
    TRUE ~ "default"
  )