Added cancel request extension method

Author: david-driscoll

src/Lsp/Protocol/General/CancelRequestExtensions.cs

@@ -0,0 +1,24 @@
+using OmniSharp.Extensions.LanguageServer.Models;
+
+// ReSharper disable CheckNamespace
+
+namespace OmniSharp.Extensions.LanguageServer.Protocol
+{
+    public static class CancelRequestExtensions
+    {
+        public static void CancelRequest(this ILanguageServer mediator, CancelParams @params)
+        {
+            mediator.SendNotification<CancelParams>("$/cancelRequest", @params);
+        }
+
+        public static void CancelRequest(this ILanguageServer mediator, string id)
+        {
+            mediator.SendNotification<CancelParams>("$/cancelRequest", new CancelParams() { Id = id });
+        }
+
+        public static void CancelRequest(this ILanguageServer mediator, long id)
+        {
+            mediator.SendNotification<CancelParams>("$/cancelRequest", new CancelParams() { Id = id });
+        }
+    }
+}

src/Lsp/Protocol/General/ICancelRequestHandler.cs

@@ -7,4 +7,4 @@ namespace OmniSharp.Extensions.LanguageServer.Protocol
 {
     [Method("$/cancelRequest")]
     public interface ICancelRequestHandler : INotificationHandler<CancelParams> { }
-}
+}