Use more portable paths in R formatter

[juliasilge]

Addresses #3690 in the same manner as we did #2194

QA Notes

This needs to be checked/tested on Windows.

Take some malformed R code like so:

fn <- function() {
  x <- 1
y <- 2
  x + y
}

And then use "Format Document" or (if selected) "Format Selection" from the command palette.

[benyamindsmith]

@juliasilge when I mapped my R keybindings I was expecting my usual ctrl + shift + a shortcut to format malformed code to work, but it does not seem to work.

[juliasilge]

@benyamindsmith I think it's likely you have a keyboard shortcut conflict:

You may want to deactivate the non-formatter one, if that is your preference. You can read more about keyboard shortcut conflicts here.