Add css for pandoc syntax highlighting themes

If your html contains code, you're likely to want to match the syntax highlighting colours with the them as a whole. So I think it makes sense for bootstraplib to provide some tools to turn pandoc themes into css. Could building from something like this:

library(jsonlite)
library(purrr)
library(dplyr)
library(farver)
library(ggplot2)

# From https://github.com/jgm/skylighting/blob/a1d02a0db6260c73aaf04aae2e6e18b569caacdc/skylighting-core/src/Skylighting/Format/HTML.hs#L117-L147
abbr <- c(
  "Keyword"        = "kw",
  "DataType"       = "dt",
  "DecVal"         = "dv",
  "BaseN"          = "bn",
  "Float"          = "fl",
  "Char"           = "ch",
  "String"         = "st",
  "Comment"        = "co",
  "Other"          = "ot",
  "Alert"          = "al",
  "Function"       = "fu",
  "RegionMarker"   = "re",
  "Error"          = "er",
  "Constant"       = "cn",
  "SpecialChar"    = "sc",
  "VerbatimString" = "vs",
  "SpecialString"  = "ss",
  "Import"         = "im",
  "Documentation"  = "do",
  "Annotation"     = "an",
  "CommentVar"     = "cv",
  "Variable"       = "va",
  "ControlFlow"    = "cf",
  "Operator"       = "op",
  "BuiltIn"        = "bu",
  "Extension"      = "ex",
  "Preprocessor"   = "pp",
  "Attribute"      = "at",
  "Information"    = "in",
  "Warning"        = "wa",
  "Normal"         = ""
)


theme <- jsonlite::read_json("https://raw.githubusercontent.com/rstudio/distill/master/inst/rmarkdown/templates/distill_article/resources/arrow.theme")

as_row <- function(x) {
  x %>%
    modify_if(is.null, ~ NA) %>%
    as_tibble()
}

styles_df <- theme$`text-styles` %>%
  purrr::map_df(as_row, .id = "name") %>%
  rename(color = `text-color`, background = `background-color`) %>%
  mutate(abbr = unname(abbr[name]))

# From https://accessible-colors.com
rel_l <- function(x) {
  scale <- function(x) {
    ifelse(x <= 0.03928, x / 12.92, ((x + 0.055) / 1.055)^2.4)
  }
  rgb <- farver::decode_colour(x) / 255
  0.2126 * scale(rgb[, 1]) + 0.7152 * scale(rgb[, 2]) + 0.0722 * scale(rgb[, 3])
}
contrast_ratio <- function(x, y) {
  x_l <- rel_l(x)
  y_l <- rel_l(y)

  (pmax(x_l, y_l) + 0.05) / (pmin(x_l, y_l) + 0.05)
}

styles_df %>% mutate(contrast = contrast_ratio(color, "#fefefe"))