Add support for alternate screen buffers

Closes #250

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Exclusive.cs

@@ -29,7 +29,7 @@ namespace Spectre.Console
         /// <returns>The result of the function.</returns>
         public static Task<T> RunExclusive<T>(this IAnsiConsole console, Func<Task<T>> func)
         {
-            return console.ExclusivityMode.Run(func);
+            return console.ExclusivityMode.RunAsync(func);
         }
     }
 }

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Screen.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IAnsiConsole"/>.
+    /// </summary>
+    public static partial class AnsiConsoleExtensions
+    {
+        /// <summary>
+        /// Switches to an alternate screen buffer if the terminal supports it.
+        /// </summary>
+        /// <param name="console">The console.</param>
+        /// <param name="action">The action to execute within the alternate screen buffer.</param>
+        public static void AlternateScreen(this IAnsiConsole console, Action action)
+        {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
+            if (!console.Profile.Capabilities.Ansi)
+            {
+                throw new NotSupportedException("Alternate buffers are not supported since your terminal does not support ANSI.");
+            }
+
+            if (!console.Profile.Capabilities.AlternateBuffer)
+            {
+                throw new NotSupportedException("Alternate buffers are not supported by your terminal.");
+            }
+
+            console.ExclusivityMode.Run<object?>(() =>
+            {
+                // Switch to alternate screen
+                console.Write(new ControlCode("\u001b[?1049h\u001b[H"));
+
+                // Execute custom action
+                action();
+
+                // Switch back to primary screen
+                console.Write(new ControlCode("\u001b[?1049l"));
+
+                // Dummy result
+                return null;
+            });
+        }
+    }
+}