Make the csharp even smaller with the C# 9 changes

[ctyar]

With top-level programs in C# 9, we can reduce the size significantly.
The result is still same as:

class A
{
    static void Main()
    {
    }
}

[dscorbett]

I think it would be nice to keep the old file as csharp-8.cs, like how there are many files for old versions of HTML.

[ctyar]

@dscorbett done.
Also, there is a new line at the end of the old file, should I remove that too?

[dscorbett]

Yes, assuming that a final newline wasn’t necessary in C# 8.

[ToastyMallows]

If we're just focusing on the .cs file itself and not the associated .csproj file, using top-level statements, here's the smallest file that I could come up with:

int a;

This file builds and runs, outputs nothing, and is 6 bytes in size.