Fix lfs.attributes() reporting paths with non-ASCII characters as non-existent on Windows

[line0]

lfs.attributes() reports any path containing a non-ASCII character as nonexistent on Windows, unless the machine is configured for the UTF-8 pseudo-ANSI Code Page (65001). Linux and macOS are not affected.

Unlike every other lfs entry point that takes a path, get_mode() hands the UTF-8 bytes it receives from Lua straight to std::filesystem instead of going through agi::fs::path, causing them to be misinterpreted as whatever encoding is specified by the system ACP.

Beyond lfs.attributes(path, "mode"), the lfs.attributes(path) signature is also affected, as it returns early when get_mode() returns nil.

This surfaced as a DependencyControl error when trying to install packages for Windows users whose account names contain non-ASCII characters: TypesettingTools/Aegisub-Motion#57

Reproduction:

script_name = "lfs.attributes Windows Unicode Bug Repro"
script_description = "lfs.attributes misses directories with non-ASCII characters."

local lfs = require("lfs")

local nonAsciiName = "aegisub-lfs-attributes-bug-repro_Müller"
local function repro()
  local tempDir = aegisub.decode_path("?temp")
  local reproDir = tempDir .. "/" .. nonAsciiName

  lfs.mkdir(reproDir)
  for entry in lfs.dir(tempDir) do
        if entry == nonAsciiName then
      aegisub.log(1, "lfs.dir lists: '%s'\n", entry)
    end
  end
  aegisub.log(1, "lfs.attributes 'mode': %s (Good: directory; Bad: nil)\n", tostring(lfs.attributes(reproDir, "mode")))
  aegisub.log(1, "lfs.attributes 'modification': %s\n", tostring(lfs.attributes(reproDir, "modification")))
  lfs.rmdir(reproDir)
end

aegisub.register_macro(script_name, script_description, repro)

[CoffeeFlux]

Filed https://developercommunity.visualstudio.com/t/C1001-in-msc1cpp:-filesystem::path-deri/11134321 and it should probably be linked in the comment. We'll have to include the workaround until we no longer want to support affected VS versions.