Implement initialized notification handler to get rid of log error

We do nothing in the handler atm but in the future we can do dynamic
registration of capabilities in it.

Author: rkeithhill

src/PowerShellEditorServices.Protocol/LanguageServer/InitializedNotification.cs

@@ -0,0 +1,29 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.PowerShell.EditorServices.Protocol.MessageProtocol;
+
+namespace Microsoft.PowerShell.EditorServices.Protocol.LanguageServer
+{
+    /// <summary>
+    /// The initialized notification is sent from the client to the server after the client received the result 
+    /// of the initialize request but before the client is sending any other request or notification to the server. 
+    /// The server can use the initialized notification for example to dynamically register capabilities. 
+    /// The initialized notification may only be sent once.
+    /// </summary>
+    public class InitializedNotification
+    {
+        public static readonly
+            NotificationType<InitializedParams, object> Type =
+            NotificationType<InitializedParams, object>.Create("initialized");
+    }
+
+    /// <summary>
+    /// Currently, the initialized message has no parameters.
+    /// </summary>
+    public class InitializedParams
+    {
+    }
+}

src/PowerShellEditorServices.Protocol/Server/LanguageServer.cs

@@ -99,6 +99,7 @@ public void Start()
             this.messageHandlers.SetEventHandler(ExitNotification.Type, this.HandleExitNotification);
 
             this.messageHandlers.SetRequestHandler(InitializeRequest.Type, this.HandleInitializeRequest);
+            this.messageHandlers.SetEventHandler(InitializedNotification.Type, this.HandleInitializedNotification);
 
             this.messageHandlers.SetEventHandler(DidOpenTextDocumentNotification.Type, this.HandleDidOpenTextDocumentNotification);
             this.messageHandlers.SetEventHandler(DidCloseTextDocumentNotification.Type, this.HandleDidCloseTextDocumentNotification);
@@ -178,6 +179,13 @@ private async Task HandleExitNotification(
             await this.Stop();
         }
 
+        private Task HandleInitializedNotification(InitializedParams initializedParams,
+            EventContext eventContext)
+        {
+            // Can do dynamic registration of capabilities in this notification handler
+            return Task.FromResult(true);
+        }
+
         protected async Task HandleInitializeRequest(
             InitializeParams initializeParams,
             RequestContext<InitializeResult> requestContext)