Support standalone language servers for R

The R module currently defaults to pkgs.rPackages.languageserver and installs languages.r.lsp.package through rWrapper. This assumes the selected server is an R package, so the option does not support a standalone executable such as Arity, which is a language server, formatter, and linter for R that I have created and maintain. It is available in nixpkgs as pkgs.arity.

Would you be open to a PR that either

1. lets the R module support both standalone language servers and the existing R languageserver package?
2. replaces the default languageserver with Arity, which won't require rWrapper to install?

Number 1 could also mean replacing the default languageserver with Arity, but still allowing users to select the existing R package if they prefer.

Arity includes formatting and linting too so it is somewhat more featureful than languageserver, but I understand if you want to keep the default as languageserver for compatibility reasons.