Check compatibility with new relase of pandoc

[htdebeer]

Pandoc has released a new version, introducing custom readers for the --from option. Check if this would work with paru, and also check custom writers, that have existed for while.

[htdebeer]

This works with paru; there's no reason to assume this would not work with pandocomatic because it just pushes the input or output formats to paru. I close this issue until evidence that suggest there is actually an issue with custom readers/writers