[Scarpet] Added ability to cancel events (#1497)

* Added ability to cancel events

* Made more events cancellable

* Added event_priority to __config

* Check for string values for performance

* Made player_finishes_using_item cancellable

* Made player_chooses_recipe cancellable

* Made player_swaps_hands cancellable

* Made player_message cancellable

* Added __on_player_placing_block (#1498)

* Added event to docs, keywords, event_test

* Added docs for event cancellation

src/main/java/carpet/script/utils/GlocalFlag.java

@@ -48,21 +48,22 @@ public class GlocalFlag extends ThreadLocal<Boolean>
         return result;
     }
 
-    public boolean runIfEnabled(Runnable action)
+    public <T> T runIfEnabled(Supplier<T> action)
     {
         synchronized (this)
         {
-            if (get() != initial) return false;
+            if (get() != initial) return null;
             set(!initial);
         }
+        T result;
         try
         {
-            action.run();
+            result = action.get();
         }
         finally
         {
             set(initial);
         }
-        return true;
+        return result;
     }
 }