Improve conversion error messages

When a conversion to an enum fails, list all the valid enum values in the error message.

Message before this commit:
> Error: heimday is not a valid value for DayOfWeek.

Message after this commit:
> Error: Failed to convert 'heimday' to DayOfWeek. Valid values are 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'

test/Spectre.Console.Cli.Tests/Data/Settings/HorseSettings.cs

@@ -0,0 +1,7 @@
+namespace Spectre.Console.Tests.Data;
+
+public class HorseSettings : MammalSettings
+{
+    [CommandOption("-d|--day")]
+    public DayOfWeek Day { get; set; }
+}