Moved the game engine into its own unit and then made a failing test for it.

game_engine.hpp

@@ -0,0 +1,22 @@
+#include <string>
+#include <map>
+
+using namespace std;
+
+class GameEngine {
+  int hit_points = 0;
+  map<string, int> damage_types{
+    {"error", 4},
+    {"warning", 1},
+    {"note", 1},
+  };
+
+  public:
+  GameEngine(int hp);
+
+  int determine_damage(string &type);
+
+  bool hit(string &type);
+
+  bool is_dead();
+};