feat: add the rows_hide() and rows_unhide() functions

[rich-iannone]

This PR adds two new functions for controlling row visibility in gt tables, mirroring the existing cols_hide() and cols_unhide() functionality for columns. rows_hide() hides rows from the rendered output while keeping them accessible for row references and expressions. rows_unhide() reveals previously hidden rows.

In this new functionality, hidden rows are excluded from the rendered table but can still be targeted in rows expressions. Footnote and style indices automatically adjust to skip hidden rows. Summary rows (summary_rows(), grand_summary_rows()) calculate from visible rows only. Row groups with all rows hidden are automatically removed from the output.

Here's an example:

library(gt)

countrypops |>
  dplyr::filter(country_name == "Brazil", year %in% 2017:2021) |>
  gt(rowname_col = "year") |>
  fmt_integer(columns = population) |>
  rows_hide(rows = c("2018", "2020")) |>
  tab_footnote(
    footnote = "Peak year in this period.",
    locations = cells_body(columns = population, rows = year == 2021)
  )

This renders a table showing only 2017, 2019, and 2021.

Here's an example that tests for footnote rendering:

countrypops |>
  dplyr::filter(country_name == "Brazil", year %in% 2017:2021) |>
  gt(rowname_col = "year") |>
  fmt_integer(columns = population) |>
  tab_footnote(
    footnote = "2017",
    locations = cells_body(columns = population, rows = year == 2017)
  ) |>
  tab_footnote(
    footnote = "2018",
    locations = cells_body(columns = population, rows = year == 2018)
  ) |>
  tab_footnote(
    footnote = "2019",
    locations = cells_body(columns = population, rows = year == 2019)
  ) |>
  tab_footnote(
    footnote = "2020",
    locations = cells_body(columns = population, rows = year == 2020)
  ) |>
  tab_footnote(
    footnote = "2021",
    locations = cells_body(columns = population, rows = year == 2021)
  ) |>
  tab_footnote(
    footnote = "2017-2018",
    locations = cells_body(columns = population, rows = year %in% c(2017, 2018))
  ) |>
  tab_footnote(
    footnote = "2018-2019",
    locations = cells_body(columns = population, rows = year %in% c(2018, 2019))
  ) |>
  tab_footnote(
    footnote = "2019-2020",
    locations = cells_body(columns = population, rows = year %in% c(2019, 2020))
  ) |>
  tab_footnote(
    footnote = "2020-2021",
    locations = cells_body(columns = population, rows = year %in% c(2020, 202))
  ) |>
  rows_hide(rows = c("2018", "2020"))

This complex footnote example shows that the multiple inclusion of footnotes avoids hidden rows and the resultant numbering/marks is correct.

Fixes: #975