Support for newer Language levels/versions

[ThaDaVos]

For example the dropdown allows at highest to select C# version 6 - latest is 11 (12 being in preview)

One thing I keep running into is the template used for the namespace - I rather use the file scoped namespace instead of the normal namespace option: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/namespace

Also, more configuration options, like not using partial would be great too!