[quarto] Add fmt_markdown() support for LaTeX output tables (data-qmd attr)

[rich-iannone]

          @rich-iannone I do think there is a related issue mentioned in quarto-dev/quarto-cli#3340 with `\QuartoMarkdownBase64{` as raw LaTeX in a cell, and **gt** not escaping it. 

Currently fmt_passthrough with escape = FALSE seems to be the solution. Though I am wondering if this should be by cell and not column.

This makes me think about some tricks we using in knitr where using I() is a way to mark some content as not to be touched.

Like

tibble::tribble(
  ~Thing, ~Citation,
  1234, I("\\QuartoMarkdownBase64{QGVxLW1hdGg=}"),
  5678, "\\QuartoMarkdownBase64{QExvdmVsYWNlMTg0Mg==}"
) |>
  gt()

The first one would go through the processing of text but not be escape_latex(), while the second would be escaped.

Anyhow, maybe not really gt-like and a latex() should work with md() or html().

To be discussed. Do you want another issue specific for LaTeX ? Or should would make this one generic about how to pass some raw content in cells ?

Originally posted by @cderv in #1487 (comment)