Clarify the behavior of `\` in a file name

The spec says that / is the directory separator. Does this mean that \ is permissible in file names? Of course on Linux that's fine, but I would guess that it's not fine on Windows.

Generally speaking, I think that @sunfishcode's comment from way back when (https://github.com/WebAssembly/WASI/issues/775#issuecomment-962343948) regarding case sensitivity also applies here: the point is to allow access to people's files, so we can't forbid names that are allowed by the FS (at least for paths relative to a root); so we have to allow for \ being a part of a file name in Linux. What would be the expected behavior for foo\bar in Windows, would \ be a separator also?

It would be useful if the spec had some kind of canonical orientation language to let idiots like me 🤓 know what kinds of consistency is aimed at with wasi:filesystem, and what kinds of divergences are expected.

Start with the filesystem specification language defining / as the directory separator and read @sunfishcode's linked comment on case sensitivity. Compare the expected treatment of \ in Linux and Windows, then clarify the specification's intended cross-platform consistency and behavior for names such as foo\bar.