NIOFileSystem: createDirectory that doesn't fail if dir already exists

[weissi]

Every createDirectory call I have is wrapped in

        do {
            try await fileSystem.createDirectory(at: logsDir, withIntermediateDirectories: true)
        } catch let error as FileSystemError where error.code == .fileAlreadyExists {
            // ok
        }

I doubt this is even correct. What I want is:

• Succeed if nothing exists
• Succeed if a directory already exists at destination
• Fail if a non-directory file exists at destination

[Lukasa]

Similarly to #3403, this is also a good idea.