FR: Auto-detect Quarto's bundled pandoc as fallback

(Potential upstream issue.)

Hi folks.

I just ran into this error on a new laptop, whilst calling some of the btw help documentation tools (e.g., btw_tool_docs_help_page) from the CLI:

Mcp error: -32603: pandoc is required and was not found (see the help page ?rmarkdown::pandoc_available).

The error is obvious enough; I didn't have pandoc installed as a "standalone" binary and available on my PATH (via my package manager). OTOH it's not the full truth, since I do have Quarto installed and this obviously bundles its own pandoc. I can imagine that other users are in a similar boat and this made me think that having a fallback option to / check for Quarto pandoc would be a useful feature.

What do you think about adding automatic detection of Quarto's bundled pandoc as a fallback when rmarkdown::pandoc_available() returns FALSE? Specifically, we could check common Quarto installation paths:

• macOS: /Applications/quarto/bin/tools/pandoc
• Windows: ~/AppData/Local/Programs/Quarto/bin/tools/pandoc
• Linux: /opt/quarto/bin/tools/pandoc

If found we could set Sys.setenv(RSTUDIO_PANDOC = <path>) to make it available to rmarkdown during this session?

Start with the CLI entry point mentioned, btw_tool_docs_help_page, and inspect how it checks rmarkdown::pandoc_available(). Verify the proposed macOS, Windows, and Linux Quarto paths, then confirm that a detected bundled pandoc is made available for the session without changing the existing error when none is found.