Add support for fake input in asciicast recordings

* Fixes a bug with `SelectionPrompt` and page size.
* Allow `IAnsiConsoleInput` to return `null`.

src/Spectre.Console/IAnsiConsoleInput.cs

@@ -12,6 +12,6 @@ namespace Spectre.Console
         /// </summary>
         /// <param name="intercept">Whether or not to intercept the key.</param>
         /// <returns>The key that was read.</returns>
-        ConsoleKeyInfo ReadKey(bool intercept);
+        ConsoleKeyInfo? ReadKey(bool intercept);
     }
 }