Enable quarto documents

**Is your feature request related to a problem? Please describe.**

I have been using HackMD with great success as a collaboration tool in academic research when use R Markdown documents to write papers. HackMD seems to recognize the .Rmd extension on R Markdown documents and imports them from GitHub without problem. However, Posit (the company that created R Markdown) recently created a second generation R Markdown called Quarto. Like R Markdown, it is a markdown format document with executable code chunks. The file suffix is .qmd which unfortunately does not seem to be recognized by HackMD when trying to import a file from GitHub.

**Describe the solution you'd like**

Please add the .qmd extension to the list of available markdown documents that one can import from GitHub.

**Describe alternatives you've considered**

There is not alternative solution that I am aware of.

**Additional context**

I teach workshops to academics on using R Markdown/Quarto and GitHub and I regularly promote HackMD as a good option for author collaboration when using R Markdown. I would like to be able to do the same for Quarto!