Add support C#

[IFierta]

Please add C# syntax support.

[tolmasky]

Hi @IFierta, would you consider submitting this mode if I were to help you get set up? I've started with the basics, but as I am not currently using C# day-to-day, I don't think I would do the Mode justice (I can certainly get basic keywords highlighting and function/class/method identification probably). However, it would be really cool to for example do proper context-aware keyword detection (for example, await is only a keyword in async functions, etc.) It would also give you the chance to take ownership of the Syntax Mode and define its direction.