dev: added step_count and health recovery

Every 15 steps the player will regain 1 HP,
unless they are at max HP. Also added commented
code for later hunger if this is to be implemented.
Played around with step # and 15 seems okay until
hunger is implemented

src/common.h

@@ -44,6 +44,7 @@ typedef struct {
   int attack;
   int defense;
   int floor;
+  int step_count;
   Item inventory[MAX_INVENTORY];
   int inventory_count;
 } Player;