Update csharp.cson, fix converter paths

[worldbeater]

Description of the Change

As you know, language-csharp grammar that is used by both Atom and Github comes from upstream. So this PR updates the grammar as there were a lot of fixes and improvements since the grammar was updated last time (dotnet/csharp-tmLanguage#114 dotnet/csharp-tmLanguage#132 dotnet/csharp-tmLanguage#113 etc.)

Python script from scripts/ folder was used to keep PCRE support, it removed dashes from group names. Manual fixes were applied according to these commits: 3b93970 15df170 17f89b8 (GitHub highlighter needs them)

Before

// Here used to be bad highlighting, but now 
// changes are merged and the code looks OK!
Span<int> arr = stackalloc [] {1, 2, 3};
static void M(in S arg);
public ref struct RefStruct { }

After

C# language syntax highlighting is improved and now supports ref struct, stackalloc, etc.

Possible Drawbacks

None.

[damieng]

Really nice piece of work thanks @worldbeater !

[worldbeater]

You are welcome @damieng! And thank you for reviewing this 🥇