Add day selector

Days/Day.cs

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+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}";
+    }
+}

Days/Day1.cs

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+using Spectre.Console;
+
+namespace AdventOfCode.Days;
+
+public class Day1 : Day
+{
+    public override int Number => 1;
+    public override string Name => "First day";
+
+    public override void Run()
+    {
+        AnsiConsole.WriteLine("Day 1 result");
+    }
+}

Days/Day2.cs

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+using Spectre.Console;
+
+namespace AdventOfCode.Days;
+
+public class Day2 : Day
+{
+    public override int Number { get; } = 2;
+    public override string Name { get; } = "Second day";
+    
+    public override void Run()
+    {
+        AnsiConsole.WriteLine("Day 2 result");
+    }
+}