bug#59490: 29.0.50; Add C# support

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Wed, 23 Nov 2022 12:38:19 -0800
> Cc: theo@thornhill.no,
>  59490@debbugs.gnu.org
> 
> Actually, another thing: it seems the mode defined is still named
> csharp-mode rather than csharp-cc-mode.

I think it is better in general to keep the name of the existing mode for
which this one is supposed to be a replacement/continuation.