Add support for Quarto assignments in R

[ttimbers]

More and more R users are using Quarto (.qmd) files instead of R Markdown (.Rmd) files because they have a lot of nice built in functionality that takes more effort to layer on in .Rmd. As such, it would be nice for ottergrader to support .qmd files. These files are extremely similar, and from reading the source code, I think the only change needed it to support either .Rmd or .qmd file extensions for R files. This can work because knitr::purl("homework.qmd") works the same as knitr::purl("homework.Rmd") - and so nothing else in the code logic needs to change I think.

Would a PR for this be welcome?

[chrispyles]

Thanks for the FR! Yes, happy to review a PR.

[ttimbers]

OK, starting working on test files first! Opened a PR where I am going to work slowly and ask for your input frequently as I am not super familiar with your code base. See #816