Add day selector

Days/Day.cs

@@ -0,0 +1,14 @@
+namespace AdventOfCode.Days;
+
+public abstract class Day
+{
+    public abstract int Number { get; }
+    public abstract string Name { get; }
+    
+    public abstract void Run();
+
+    public override string ToString()
+    {
+        return $"Day {Number}: {Name}";
+    }
+}