fix: ignore file detection when directory contains period

[JSanchezIO]

I was using CSharpier in a project whose directory name contained a period, e.g., /home/jsanchezio/Documents/Repos/My.Project/, and despite having a .csharpierignore file, it would still report files excluded by the ignore as unformatted. I was running the command as dotnet csharpier --check . After some investigation, it turns out fileSystem.Path.GetExtension(directoryOrFile) would return the path after the period, e.g., .Project. This change alters the command to instead check if the directoryOrFile is a directory that exists and reacts accordingly.

After I discovered the issue, I renamed my project's folder name and all worked well. However, in CI environments there is no guarantee that a . won't be in the path, so I figured it was best to handle it within CSharpier.

[shocklateboy92]

LGTM. Thanks!