Add copyright

Author: Rob Holt

src/PowerShellEditorServices.Engine/Services/TextDocument/Handlers/DocumentHighlightHandler.cs

@@ -1,3 +1,8 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
 using Microsoft.Extensions.Logging;
 using Microsoft.PowerShell.EditorServices.Symbols;
 using OmniSharp.Extensions.LanguageServer.Protocol.Models;