Add net5.0 target framework

src/Spectre.Console/Extensions/TextWriterExtensions.cs

@@ -0,0 +1,21 @@
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+
+namespace Spectre.Console.Internal
+{
+    internal static class TextWriterExtensions
+    {
+        [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
+        public static bool IsStandardOut(this TextWriter writer)
+        {
+            try
+            {
+                return writer == System.Console.Out;
+            }
+            catch
+            {
+                return false;
+            }
+        }
+    }
+}