Incorrect api used for sys_read_dir on windows

On windows, neko uses [`FindFirstFileA` for `sys_read_dir`](https://github.com/HaxeFoundation/neko/blob/c41bdd4f0600680fc23ad6b05bd30e3c398cc6cc/libs/std/sys.c#L527). This returns ANSI encoded strings (not to be confused with ASCII) which means that any non-ascii characters are not compatible with utf8, so they can cause issues when the strings from this function are passed to other places.

We should instead use the `FindFirstFileW` variant which gives utf-16 encoded strings, which can then be converted to valid utf-8.

Similarly, we should replace any use of ANSI variants of the windows api.

This way the behaviour will match non-windows platforms and be more compatible.

Start in libs/std/sys.c around line 527 and inspect the Windows API calls used by sys_read_dir, then identify other ANSI variants covered by the issue. Done means directory strings and other affected paths handle non-ASCII characters as UTF-8 and match non-Windows behavior.

Written by the indexing model from the issue text.