Use CSC to Run C# Programs

The Code Runner extension currently uses [ScriptCS](http://scriptcs.net/) to run C# programs, but I can't get it to work properly (possibly due to its dependencies), and have instead resorted to running CSC with the following macro:

`cd $dir && csc $fileName && $dir$fileNameWithoutExt`

This seems to work very well, and is similar to the current macros for C and C++.

Considering that CSC is probably the most commonly-used compiler for C#, I recommend that the developers for this extension update the `package.json` file to include this macro mapping.