Review UX when interacting with `io.fs`

[pjbgf]

For next major release, the user experience when interacting between go-billy and io.FS should be more seamless.
Amongst other things, go-billy's interfaces could be based of io.FSs (e.g. ReadDirFile, ReadDirFS, etc).

[pjbgf]

@infogulch thanks for raising the point on os.Root which is in the upcoming Go 1.24. The behaviour below would be compatible with ChrootOS but not BoundOS:

Methods on Root will follow symbolic links, but symbolic links may not reference a location outside the root. Symbolic links must not be absolute.

Support to this would most certainly be on go-billy v6 or above.