Support ANSI colour output with `downlit`

This issue is based on the discussion in https://github.com/quarto-dev/quarto-cli/discussions/3717. Just reporting here so it's easier to follow.

It would be nice if Quarto supported ANSI colour output with downlit. Currently Quarto uses downlit for code linking when code-link: true, but this doesn't make colours work.

I thought it would work based on my experiences with pkgdown, but after some discussion it sounds like pkgdown does some additional processing with fansi to support colour output (See https://github.com/quarto-dev/quarto-cli/discussions/3717#discussioncomment-4459352). At this time, if you do need colour output in Quarto, you can get it use the fansi knitr hooks for converting ANSI highlighting to HTML highlighting. See https://github.com/quarto-dev/quarto-cli/discussions/3717#discussioncomment-4459352.

One thing that is nice about this approach with fansi is that you can turn the hook on and off throughout a document, in case you only want colour output for certain parts. For example, in this blog post I have fansi hooks on in the body of the post, but turned them off for the appendix so the output from sessioninfo::session_info() wouldn't be coloured.

If Quarto does support colour output in the future, it would be nice to make it a chunk option (e.g., colour-output: true/false) rather than a document-wide option like code-link so it can be used selectively when needed.

Here's a reprex copied from the discussion that can be used for testing.

---
title: "Reprex"
format:
  html:
    code-link: true
---

```{r}
#| collapse = TRUE
cat(cli::col_red("This is red"), "\n")
cat(cli::col_blue("This is blue"), "\n")

message(cli::col_green("This is green"))

warning(cli::style_bold("This is bold"))
```