Suppport C# alias for c#

Motivation
Please explain the problem you're having/why you propose this feature. Ex. I'm always frustrated when [...]

Support c# alias for c# language.

The basic motivation is github support this, so it would be better if we can get same behavior with prism result and github result for markdown files as docs:

GitHub result:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
    static void Main(string[] args)
    {
    }
    }
}

Raw content:

```c#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
    static void Main(string[] args)
    {
    }
    }
}
```

Description
A clear and concise description of what you want to happen.

Alternatives
A clear and concise description of any alternative solutions or features you've considered, if applicable.

Start by locating Prism's C# grammar and the language-alias handling used for Markdown code fences. Compare c# with the existing C# identifier and verify that both produce the same highlighting result; done means the alias is accepted consistently with GitHub's behavior.