Add support for raw string literals for C#11

[evgeniykim]

Would be nice to have highlighting not only in verbatim string literals but even in new raw string literals (https://devblogs.microsoft.com/dotnet/csharp-11-preview-updates/#raw-string-literals):
Extension works:

Extension does not work:

[ilengyel]

Note it is also very picky about the order of interpolation marker and new line: