[Maintenance] [th02] Start a new header for player performance metrics

All of these are scalar values with at most a dependency on
`platform.h`. Since these item-related values are mainly read by
score/skill calculation code, it makes sense to declare them in a
dedicated header along with the upcoming metrics.

Part of P0279, funded by Yanga and Blue Bolt.

th02/main/item/item.cpp

@@ -15,6 +15,7 @@ extern "C" {
 }
 #include "th02/main/entity.hpp"
 #include "th02/main/playfld.hpp"
+#include "th02/main/playperf.hpp"
 #include "th02/main/score.hpp"
 #include "th02/main/scroll.hpp"
 #include "th02/main/hud/hud.hpp"

th02/main/item/item.hpp

@@ -15,12 +15,6 @@ enum item_type_t {
 // items. Used for recharging power after using a continue after a Game Over.
 extern unsigned int item_bigpower_override;
 
-// Increases by varying amounts depending on how well items are collected,
-// decrements for every 16 dropped items.
-extern int item_skill;
-
-extern int point_items_collected;
-
 // Spawns the Game Over item set on the next call to items_miss_add().
 // ZUN bloat: Both turning this into a parameter or hardcoding the condition
 // (as TH04 and TH05 do it) would have been better than this.

th02/main/playperf.hpp

@@ -0,0 +1,8 @@
+// Player performance metrics
+// --------------------------
+
+// Increases by varying amounts depending on how well items are collected,
+// decrements for every 16 dropped items.
+extern int item_skill;
+
+extern int point_items_collected;