Add an option in setup action to put bundled Pandoc in PATH

[cderv]

cc @hadley

With something like this

steps:
    - uses: quarto-dev/quarto-actions/setup@v2
      env:
        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
      with:
        add-pandoc-to-path: true

As we know where Pandoc is in the installed folder, we could add it to $GITHUB_PATH to make it available also outside of quarto rendering.

[cderv]

Another way to help others use the bundled quarto version would be to return as an action output the path to Pandoc binary.

This way it could be set by others in a later step to PATH or to an env var (like RSTUDIO_PANDOC for rmarkdown use only)

This would be default output - no flag needed.

[hadley]

I think it would be more helpful to just set the path — it would be nice to have one action that allows to you use pandoc in both quarto and RMarkdown docs.