add default .editorconfig and apply dotnet format

[xavierpinho]

• Removes a garbage file, viz. abc def/sample.cs
• Adds an .editorconfig taken from Microsoft Learn with conventional default settings. EDIT: one difference (see 8501a93) -- prefer \n instead of \r\n for end of line. Maybe there's a good reason to use \r\n, but I don't know 🤷‍♂️
• Applies dotnet format
• Removes some unused imports
• Adds a build job for checking that the code is formatted, similarly to what we have on joern with scalafmt