Use builtin path manipulation functions

[kkanden]

hi,
i've often encountered issues with incorrect windows path handling and in the codebase i see many functions to handle them (and sometimes are inconsistently used, e.g. as fixed in #599). neovim has builtin vim.uv and vim.fs (specifically vim.fs.normalize, which on windows and mingw change backward slashes to forward slashes) functions which ensure compatibility on all platforms and allow us to not reinvent the wheel ;)

if this is something of interest, i could write up a PR to change path handling to builtin equivalents (or improvements) wherever it makes sense.

[kkanden]

some examples i found that could use the improvement:
lua/r/config.lua#884:

    if config.is_windows then
        config.rnvim_home = utils.normalize_windows_path(config.rnvim_home)
        config.uservimfiles = utils.normalize_windows_path(config.uservimfiles)
    end

could be rewritten as:

        config.rnvim_home = vim.fs.normalize(config.rnvim_home)
        config.uservimfiles = vim.fs.normalize(config.uservimfiles)

(vim.fs.normalize detects whether the user is on windows)

there is also inconsistency with how paths are joined.

• lua/r/config.lua#894:

    elseif config.is_windows and vim.env.APPDATA then
        config.compldir = vim.fn.expand(vim.env.APPDATA) .. "\\R.nvim"

• lua/r/config.lua#911:

        if config.is_windows then
            if vim.env.TMP and vim.fn.isdirectory(vim.env.TMP) ~= 0 then
                config.tmpdir = vim.env.TMP .. "/R.nvim-" .. config.user_login

those could be replaced with vim.fs.joinpath (which also correctly handles windows path with changing the slashes as above)

[jalvesaq]

This is a PR that we need. The release of 1.0.0 is scheduled for July 05. Could you make the PR before that date? I would be able to try it only on Linux, but we have people on Windows who hopefully will be able to review your PR.

R.nvim was translated from VimScript, and it still has code that could be optimized with Lua-specific functions...

[kkanden]

i think i can get it done by the end of the week. my work laptop is on windows so i could test it there if no one else volunteers. i could make commit the changes by file/section/functionality to make review and debugging easier. i'll put up a draft pr soon :)