Check `VFS.DirList` path slash consistency (incl. for desync)

1) Damgam claims that he can sometimes reproduce `VFS.DirList` returning `\` instead of `/` for files (apparently it's for additional read folders outside Spring's normal tree, eg. `C:\My Documents\bla` absolute paths on Windows as opposed to the usual `/bla` relative)
2) It should be possible to make Spring run synced archives from arbitrary paths outside of its dir (via `spring.exe --game=C:\My Documents\bar.sd7`).
3) It looks like nothing blocks such paths ( https://github.com/Beyond-all-Reason/spring/blob/BAR105/rts/Lua/LuaVFS.cpp#L265 looks like it could, though that just blocks absolute paths which may not map 1:1 to slash type)

If you can get a synced callout to return different values depending on platform, that's a ticking desync bomb. I haven't tried reproducing yet but this is may be worth checking at some point.

Different slashes are also an annoyance. It would be good if Spring encapsulated platform differences and guaranteed `/` as dir separator from `VFS.DirList` natively (this would also remove the potential for desync, anyway). This is mostly a low priority request because I haven't yet reproduced the desync and otherwise it's not too impactful.

Start with VFS.DirList and the LuaVFS.cpp code around line 265, then reproduce the reported behavior on Windows and another platform using an external path such as C:\My Documents\bar.sd7. Check whether synced callouts can return platform-dependent slash values and whether arbitrary external archive paths work; done means the behavior is verified and the requested path and separator guarantees are covered.

Written by the indexing model from the issue text.