[`pdfviewer`] Add support for or make default: `vim.ui.open`

[Aman9das]

pdfviewer is hardcoded to zathura currently, but that does not support flatpaks for example, even if zathura is installed.

The neovim lua way of opening files and urls is [vim.ui.open](https://neovim.io/doc/user/lua.html#vim.ui.open()) and it opens pdf files in the system pdf viewer quite well. It is the way gx works too since neovim 0.10

Kindly make it possible to make vim.ui.open the pdfviewer, maybe by setting the string to "vim" or even better making it the default.

[PMassicotte]

Duplicated of #144

[jalvesaq]

@Aman9das, could you, please, report if https://github.com/R-nvim/R.nvim/pull/144 works for you, and what's your operating system?

[Aman9das]

Works quite well! I am on NixOS (linux)