deactivate extension about yaml metadata block for pandoc 2.13+

[cderv]

This fixes #375 on reprex side fro now

As explained in #375 (comment), we need to make a fix for Pandoc's change in rmarkdown but I am not sure reprex would benefit from it directly because of the way the reprex_document() format is built. By not using the variant= argument in md_document(), any variant specific logic for gfm inside rmarkdown::md_document() would not be applied.

For a quick fix on reprex side, I have just added a switch to get Pandoc's extension deactivated for version 2.13 and above. This extension yaml_metadata_block has been activated by default in Pandoc now. This change in reprex does not require a new rmarkdown version.

I did not add tests for now. I wanted to wait for your guidance about how you would like it to be in your test organization. And also because tests in reprex that runs Pandoc are currently already failing with recent Pandoc versions. However, you don't test several Pandoc versions for now in GHA, only 2.7.3 (the default for r-lib/actions/setup-pandoc).

Happy to discuss it further and work on another way to fix it (with a fix in rmarkdown and a change in reprex)

[krlmlr]

Confirming that this fixes my problems with pandoc 2.13 on my M1 mac, CC @jennybc.

[jennybc]

Thanks!