[feature]: Add support for R Markdown (`*.rmd`)

[Antyos]

If possible, it would be nice to have rewrap support for R Markdown (*.rmd) files (See: R). It should be a easy implementation, as I think the syntax for R markdown is still basically just markdown, just using the rmd language id.

I can try to submit a PR I can get a bit of guidance where I would implement the R Markdown definition.

[rushgeo]

Looks like *.qmd support is also needed, which should be about the same as *.rmd.

[billdenney]

I just found a workaround (that may be the right solution overall).

• Open a .Rmd file
• Press Ctrl-K, release, then press the letter m
• That opens the "set language mode" dialog
• Then I chose "configure file association for ',Rmd'"
• I chose markdown

And, rewrap worked.

My guess is that this is something that should be handled in Positron.