Automate input filenames

One less thing to replace from the template.

src/23.cs

@@ -17,7 +17,7 @@ internal class Day23 : Day
 
     internal override void Parse()
     {
-        var lines = Parsing.ReadAllLines("23").ToImmutableList();
+        var lines = Parsing.ReadAllLines($"{GetDayNum()}").ToImmutableList();
         for (int row = 0; row < lines.Count; row++)
         {
             var line = lines[row];

src/Day.cs

@@ -47,6 +47,16 @@ internal abstract class Day : IDisposable
         }
     }
 
+    internal int GetDayNum()
+    {
+        if (int.TryParse(GetType().Name["Day".Length..], out int dayNum))
+        {
+            return dayNum;
+        }
+
+        return -1;
+    }
+
     internal virtual void Parse() { }
     internal virtual string Part1() { return string.Empty; }
     internal virtual string Part2() { return string.Empty; }

src/Template.cs

@@ -5,7 +5,7 @@ internal class DayTemplate : Day
 {
     internal override void Parse()
     {
-        var lines = Util.Parsing.ReadAllLines("##");
+        var lines = Util.Parsing.ReadAllLines($"{GetDayNum()}");
     }
 
     internal override string Part1()