Add option to change fill color of nanoplot data area (bounded to data points)

[rich-iannone]

The fill color for the area that bounds the data points in line plot type of nanoplot can now be changed through this PR. The default value is "#FF0000" ("red"), which is unchanged from before. This can be modified via nanoplot_options()'s data_area_fill_color argument.

Here is an example of this:

library(gt)
library(tidyverse)

illness |>
  dplyr::slice_head(n = 10) |>
  gt(rowname_col = "test") |>
  tab_header("Partial summary of daily tests performed on YF patient") |>
  tab_stubhead(label = md("**Test**")) |>
  cols_hide(columns = c(starts_with("norm"), starts_with("day"))) |>
  fmt_units(columns = units) |>
  cols_nanoplot(
    columns = starts_with("day"),
    new_col_name = "nanoplots",
    new_col_label = md("*Progression*"),
    options = nanoplot_options(data_area_fill_color = "lightgreen") 
  ) |>
  cols_align(align = "center", columns = nanoplots) |>
  cols_merge(columns = c(test, units), pattern = "{1} ({2})") |>
  tab_footnote(
    footnote = "Measurements from Day 3 through to Day 8.",
    locations = cells_column_labels(columns = nanoplots)
  )

Fixes: #1521