[quarto] disable_processing options should opt-out fmt_markdown specific output for quarto

[cderv]

---
title: "Test"
format: html
keep-md: true
---

```{r}
library(gt)
data.frame(A = "Text", B = '<em class="special">Text</em>') |> 
  gt() |> 
  tab_options(quarto.disable_processing = TRUE) |> 
  fmt_markdown(B)
```

You can see the weird output in the column. This is related to

• [quarto] data-qmd content should be escaped at least on double quotes #1487

and found while looking at

• Thoughts on why my gt output inside a qmd differ from it outside a qmd? quarto-dev/quarto-cli#7493

Content produced is still quarto specific

<div data-qmd="<em class="special">Text</em>">

but disabling quarto processing should have prevented inserting the data-qmd attributes.

There are other way to achieve that, but still I believe data-qmd insertion is tied to quarto table processing. So deactivating it when option is present seems better.

What works

fmt_passthrough() works though.

```{r}
library(gt)
data.frame(A = "Text", B = '<em class="special">Text</em>') |> 
  gt() |> 
  tab_options(quarto.disable_processing = TRUE) |> 
  fmt_passthrough(B, escape = FALSE)
```

[rich-iannone]

This should be solved by fixing #1487.