Make YesNtInterpreter usable as a library and add AddStatement method to be able to define custom statement

YesNt.Interpreter/Program.cs

@@ -1,14 +1,22 @@
-using System;
+using System;
 
 using YesNt.Interpreter.Runtime;
 
-if (args.Length == 1)
+namespace YesNt.Interpreter;
+
+internal static class Program
 {
-    YesNtInterpreter interpreter = new YesNtInterpreter();
-    interpreter.Initialize();
-    interpreter.Execute(args[0]);
+    internal static void Main(string[] args)
+    {
+        if (args.Length == 1)
+        {
+            YesNtInterpreter interpreter = new YesNtInterpreter();
+            interpreter.Initialize();
+            interpreter.Execute(args[0]);
+        }
+        else
+        {
+            Console.WriteLine("No path specified!");
+        }
+    }
 }
-else
-{
-    Console.WriteLine("No path specified!");
-}