Add a useful helper method

Author: davidwengier

src/Tools/ExternalAccess/Razor/ITextBufferExtensions.cs

@@ -0,0 +1,18 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.CodeAnalysis.Text;
+using Microsoft.VisualStudio.Text;
+
+namespace Microsoft.CodeAnalysis.ExternalAccess.Razor;
+
+internal static class ITextBufferExtensions
+{
+    public static bool TryGetTextDocument(this ITextBuffer textBuffer, [NotNullWhen(true)] out TextDocument? textDocument)
+    {
+        textDocument = textBuffer.CurrentSnapshot.AsText().GetOpenTextDocumentInCurrentContextWithChanges();
+        return textDocument is not null;
+    }
+}