Add `.FileNameSafe()`

Add the following extension to StringUtils:
```cs
public static string FileNameSafe(this string s,
                                  Func<string, string>? replacer = (x) => "_",
                                  OperatingSystem? os = OperatingSystem.Current);
```

i think i wrote something like this before but hardcoded for Windows. Not too hard to write from scratch, though.
