fs.fnl: Support platforms which appear to not use luajit

This patch simply updates the path-sep logic to follow the code referenced in the comment. This is required for aniseed to function correctly in my aarch64 ubuntu virtual machine.

Ohh interesting. So then you can require some of this outside of nvim / luajit? I'll take a look when I get home this evening.

Hmm am I right in saying this alternate method you added works for JIT too? Why don't we just use (package.config:sub 1 1) for all environments?

Can someone run package.config:sub(1, 1) on Windows for me and tell me if it returns \?

Unfortunately I don't have a windows machine setup to test.

This patch was mainly to support gettng aniseed working in my ubuntu 20.04 vm, which I think had an old version of neovim pre luajit (~v0.4.3). I ended up compiling neovim from source and it solved the problem (v0.8.0). All depends on if you want to support ancient neovims :)

I think I'll close this! I'm pretty sure I depend on other features that require you to have a newer neovim, maybe around floating windows which are pretty core to the UX. So for now, I'll close this. But if I get people raising issues about this same thing where everything else should be supported I'll revisit this change. Thank you for the submission!

@Olical I think this PR would fix a bug I'm seeing trying to run Conjure on a Mac M1 inside Docker for Ubuntu 2210, where neovim 0.7.2 comes with Lua not LuaJIT because LuaJIT isn't supported on aarch64 (ie arm64).

:version                                                                                                                                                       
NVIM v0.7.2                                                                                                                                                    
Build type: Release                                                                                                                                            
Lua 5.1                                                                                                                                                        
Compiled by [email protected]                                                                                                                        
                                                                                                                                                               
Features: +acl +iconv +tui                                                                                                                                     
See ":help feature-compile"                                                                                                                                    
                                                                                                                                                               
   system vimrc file: "$VIM/sysinit.vim"                                                                                                                       
  fall-back for $VIM: "/usr/share/nvim"                                                                                                                        
                                                                                                                                                               
Run :checkhealth for more info                     

The bug I'm seeing is the following (when opening a Clojure file for example)

                    Error detected while processing FileType Autocommands for "clojure":                                                                       
E5108: Error executing lua ...me/vu/.vim/bundle/conjure/lua/conjure/aniseed/fs.lua:50: attempt to index global 'jit' (a nil value)                             
stack traceback:                                                                                                                                               
        ...me/vu/.vim/bundle/conjure/lua/conjure/aniseed/fs.lua:50: in main chunk                                                                              
        [C]: in function 'require'                                                                                                                             
        ....vim/bundle/conjure/lua/conjure/aniseed/autoload.lua:19: in function 'ensure'                                                                       
        ....vim/bundle/conjure/lua/conjure/aniseed/autoload.lua:28: in function <....vim/bundle/conjure/lua/conjure/aniseed/autoload.lua:27>                   
        ...u/.vim/bundle/conjure/lua/conjure/aniseed/fennel.lua:20: in function 'sync_rtp'                                                                     
        ...u/.vim/bundle/conjure/lua/conjure/aniseed/fennel.lua:43: in function 'impl'                                                                         
        /home/vu/.vim/bundle/conjure/lua/conjure/client.lua:62: in function </home/vu/.vim/bundle/conjure/lua/conjure/client.lua:61>                           
        (tail call): ?                                                                                                                                         
        /home/vu/.vim/bundle/conjure/lua/conjure/client.lua:167: in function 'get'                                                                             
        /home/vu/.vim/bundle/conjure/lua/conjure/client.lua:180: in function </home/vu/.vim/bundle/conjure/lua/conjure/client.lua:179>                         
        (tail call): ?                                                                                                                                         
        [string ":lua"]:1: in main chunk