Add support for FastEndpoints about csharpextensions

[KreativJos]

Hey
Thank you for submitting your request.
I do not have the ambition to move this extension up to a Visual studio-type 'New...' menu.
It might be interesting to create an extension to create new items by template, with importable (user-creatable) templates (like visual studio, but more plain). It might even already exist?
It would even be possible to use this feature of vscode - #36

[pm4rcin]

@KreativJos I have found an interesting thing that could be used for other templates as well and greatly reduce required amount of code and the only downside(?) is that it would depend on dotnet-sdk. We could use custom templates and execute dotnet commands e.g. in console when adding file graphically. Check how Code Butler does similiar thing. There's also vscode-solution-explorer which does it from console.