Core: Cleanup headers in core/config

- `MainLoop` now forward-declared in `OS`

core/config/engine.h

@@ -30,10 +30,13 @@
 
 #pragma once
 
-#include "core/os/main_loop.h"
-#include "core/string/ustring.h"
+#include "core/string/string_name.h"
+#include "core/templates/hash_map.h"
 #include "core/templates/list.h"
 
+class Object;
+class Dictionary;
+
 template <typename T>
 class TypedArray;
 

core/config/project_settings.h

@@ -30,7 +30,7 @@
 
 #pragma once
 
-#include "core/object/class_db.h"
+#include "core/object/object.h"
 #include "core/templates/rb_map.h"
 
 template <typename T>