Support .Rmarkdown as .Rmd

[lorenzwalthert]

For blogdown, you can use .Rmarkdown instead of .Rmd to generate tocs and other things. Reference: https://bookdown.org/yihui/blogdown/output-format.html. This format should be treated as equivalent to .Rmd by styler.

[lorenzwalthert]

Not sure if this file format should just be treated as Rmd. This would avoid some overhead in the code but make it impossible for the user to distinguish between Rmd and Rmarkdown, e.g. in style_file(..., filetype = ).

[lorenzwalthert]

Not sure this has a high priority anymore, since {hugodown}, the successor of {blogdown}, does not use .Rmarkdown.