fix: put the stats in an internal list

It'll allow us later to even drop the internal list and replace it by
automatic computation

sonic-radiance.love/game/abstractmobs/statmanager.lua

@@ -16,8 +16,9 @@ function StatManager:new(owner)
 end
 
 function StatManager:setStats(stats)
+    self.list = {}
     for key, value in pairs(stats) do
-        self[key] = value
+        self.list[key] = value
     end
 end
 
@@ -26,7 +27,7 @@ function StatManager:get(statname)
 end
 
 function StatManager:computeStat(statname)
-    return self[statname]
+    return self.list[statname]
 end
 
 return StatManager