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"))

Start with the R prototype in the issue body and compare its class mapping with the referenced Skylighting HTML formatter and the distill arrow.theme example. Determine the project entry point for Bootstrap styling before implementing the proposed theme-to-CSS tools. Done means pandoc syntax-highlighting themes can produce CSS that matches the surrounding Bootstrap theme.