Hi Yuan, Jostein and others.

Added is support for C# in Emacs.  This is the original C# mode as found
on ELPA [0], as well as support for the new tree sitter mechanisms.

I made two new modes, 'csharp-cc-mode' and 'csharp-ts-mode'. The cc-mode
variant has that name as to not conflict with the ELPA package.

Please shoot comments at me, and I'll fix them.

Hope you like it,

Theodor

[0]: https://github.com/emacs-csharp/csharp-mode