Two column layout with high level header

[mrcaseb]

This is a more or less special feature I see people asking for in the nflverse discord. The main idea is to allow a high level tab header in the two column layout.
The problem here is formatting. We can either ask the user to set all the html styles themselves or try to help via gt. So in this PR, the function extracts tab header information (including all of the styling) from the header of one of the two tables. This allows the user to do all of the styling through gt.

Example

library(gt)
my_cars <- mtcars %>%
  dplyr::mutate(row_n = dplyr::row_number(), .before = mpg) %>%
  dplyr::select(row_n, mpg:drat)

tab1 <- my_cars %>%
  dplyr::slice(1:16) %>%
  gt() %>%
  gtExtras::gt_color_rows(columns = row_n, domain = 1:32) |> 
  gtExtras::gt_theme_538()

tab2 <- my_cars %>%
  dplyr::slice(17:32) %>%
  gt() %>%
  gtExtras::gt_color_rows(columns = row_n, domain = 1:32) |> 
  gt::tab_header(
    "A LONG AND VERY INFORMATIVE TITLE", "MY SUBTITLE IS ALSO VERY COOL"
  ) |> 
  gtExtras::gt_theme_538() |> 
  gt::tab_options(
    heading.background.color = "gray"
  )

listed_tables <- list(tab1, tab2)

Current implementation

gt_two_column_layout(
  listed_tables, output = "save",
  filename = "basic-two-col.png",
  vwidth = 700, vheight = 620
)

With Header

gt_two_column_layout(
  listed_tables, output = "save",
  filename = "basic-two-col-title.png",
  vwidth = 600, vheight = 620,
  tab_header_from = "table2"
)

[mrcaseb]

I forgot to add this uses xml2 to extract styling information from raw gt html.

xml2 is already listed in Suggests but I think it's a strong reverse dependency anyway.

[mrcaseb]

There is also room for improvements here. For example high level title and table level subtitles.

[mrcaseb]

gt writes the table id into the selectors of the stylesheet. We extract that id here to replace it with our own id in the next step.

[mrcaseb]

This is necessary to apply the style to our top level heading

[mrcaseb]

The table selector sets font. Since we put the header in div tags we replace table with div in the style sheet.

[mrcaseb]

xml2 returns NA if the text is missing. We return NULL to drop it entirely from the html.

[jthomasmock]

Thanks @mrcaseb ! Going to merge and then we can iterate