Support C# 7 and 8 pattern matching

[nickspoons]

We currently highlight this incorrectly:

if (args.Length == 0 || args[0] is "help" or "-h" or "--help")

https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7#pattern-matching
https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-8#more-patterns-in-more-places
https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching