fix compilation errors for fs and fs.Dir

[JustinBraben]

Fixes the std.fs related compiler errors mentioned in #20505

[linusg]

https://ziglang.org/documentation/master/std/#std.mem.span seems like the better choice for this

[JustinBraben]

Ok! I can rework it.

[JustinBraben]

Worth mentioning I got the original mem.sliceTo(path_w, 0) from the function isAbsoluteWindowsW , which is asserted before most of these W calls.

pub fn isAbsoluteWindowsW(path_w: [*:0]const u16) bool {
    return isAbsoluteWindowsImpl(u16, mem.sliceTo(path_w, 0));
}

Should that be changed to:

pub fn isAbsoluteWindowsW(path_w: [*:0]const u16) bool {
    return isAbsoluteWindowsImpl(u16, mem.span(path_w));
}

Ok if that is outside the scope for this PR though

[JustinBraben]

Ok landed on this

pub fn symLinkAbsoluteW(
    target_path_w: [*:0]const u16,
    sym_link_path_w: [*:0]const u16,
    flags: Dir.SymLinkFlags,
) !void {
    assert(path.isAbsoluteWindowsW(target_path_w));
    assert(path.isAbsoluteWindowsW(sym_link_path_w));
    return windows.CreateSymbolicLink(null, mem.span(sym_link_path_w), mem.span(target_path_w), flags.is_directory);
}

[JustinBraben]

@linusg I switched the sliceTo to mem.span . Think it's ready for another look.