Render LaTeX formulas in md() when target is HTML (standalone/JS-free via katex)

[rich-iannone]

This PR allows for the use of LaTeX formulas between $ and $$ delimiters when rendering an HTML table (but not in the Quarto rendering environment). This requires katex as an optional dependency, which makes the rendering of formula text dependency-free in the output (advantage is that formulas could work in environments where script tags aren't allowed, like email).

Here is an example demonstrating that fmt_markdown() and md() can handle LaTeX formulas:

library(gt)
library(tidyverse)

dplyr::tibble(
  idx = 1:5,
  l_time_domain =
    c(
      "$$1$$",
      "$${{\\bf{e}}^{a\\,t}}$$",
      "$${t^n},\\,\\,\\,\\,\\,n = 1,2,3, \\ldots$$",
      "$${t^p}, p > -1$$",
      "$$\\sqrt t$$"
    ),
  l_laplace_s_domain =
    c(
      "$$\\frac{1}{s}$$",
      "$$\\frac{1}{{s - a}}$$",
      "$$\\frac{{n!}}{{{s^{n + 1}}}}$$",
      "$$\\frac{{\\Gamma \\left( {p + 1} \\right)}}{{{s^{p + 1}}}}$$",
      "$$\\frac{{\\sqrt \\pi }}{{2{s^{\\frac{3}{2}}}}}$$"
    )
) |>
  gt(rowname_col = "idx") |>
  fmt_markdown() |>
  cols_label(
    l_time_domain = md(
      "Time Domain<br/>$\\small{f\\left( t \\right) =
      {\\mathcal{L}^{\\,\\, - 1}}\\left\\{ {F\\left( s \\right)} \\right\\}}$"
    ),
    l_laplace_s_domain = md(
      "$s$ Domain<br/>$\\small{F\\left( s \\right) =
      \\mathcal{L}\\left\\{ {f\\left( t \\right)} \\right\\}}$"
    )
  ) |>
  tab_header(
    title = md(
      "A (Small) Table of Laplace Transforms &mdash; $\\small{{\\mathcal{L}}}$"
    ),
    subtitle = md(
      "Five commonly used Laplace transforms and formulas.<br/><br/>"
    )
  ) |>
  cols_align(align = "center") |>
  opt_align_table_header(align = "left") |>
  cols_width(
    idx ~ px(50),
    l_time_domain ~ px(300),
    l_laplace_s_domain ~ px(600)
  ) |>
  opt_stylize(
    style = 2,
    color = "gray",
    add_row_striping = FALSE
  ) |>
  opt_table_outline(style = "invisible") %>%
  tab_style(
    style = cell_fill(color = "gray95"),
    locations = cells_body(columns = l_time_domain)
  ) |>
  tab_options(
    heading.title.font.size = px(32),
    heading.subtitle.font.size = px(18),
    heading.padding = px(0),
    footnotes.multiline = FALSE,
    column_labels.border.lr.style = "solid",
    column_labels.border.lr.width = px(1)
  )

A snapshot test was added to test the insertion of LaTeX formulas in more locations (e.g., footnotes, source notes, etc.). It also tests that multiple inline and display-mode formulas can be processed in a single piece of Markdown text.

Fixes: #375
Fixes: #616
Fixes: #1163