fix: Correct path.sep for POSIX LuaJIT consumers (#409)

The existing path.sep logic tested explicitly for Linux, OSX, or BSD
OSes as queried through LuaJIT (if present).  This left out other POSIX
OSes, which will almost certainly use the UNIX-style forward slash,
rather than backslash.

Since Windows is the odd one out when it comes to path separators, using
it for the comparison should provide better coverage long-term.

Author: pfmooney

lua/plenary/path.lua

@@ -21,7 +21,7 @@ path.home = vim.loop.os_homedir()
 path.sep = (function()
   if jit then
     local os = string.lower(jit.os)
-    if os == "linux" or os == "osx" or os == "bsd" then
+    if os ~= "windows" then
       return "/"
     else
       return "\\"