Add day selector

Program.cs

@@ -0,0 +1,18 @@
+
+using System.Reflection;
+using AdventOfCode.Days;
+using Spectre.Console;
+
+var days = Assembly.GetAssembly(typeof(Day))!.GetTypes()
+    .Where(t => t.IsAssignableTo(typeof(Day)) && t.GetConstructor(Type.EmptyTypes) != null && !t.IsAbstract)
+    .Select(t => (Day)Activator.CreateInstance(t)!);
+
+var select = new SelectionPrompt<Day>()
+    .Title("[cyan]Select a [yellow]day[/] to run:[/]")
+    .AddChoices(days);
+
+var selectedDay = AnsiConsole.Prompt(select);
+
+AnsiConsole.MarkupLine($"[cyan]Running [yellow]{selectedDay}[/]...[/]\n");
+
+selectedDay.Run();