GDExtension: Add icon path to class creation info

Allows GDExtensions to set the icon of a class programmatically, which is useful for extensions that don't use a `.gdextension` file.

core/extension/gdextension_interface.h

@@ -368,6 +368,7 @@ typedef struct {
 	GDExtensionBool is_abstract;
 	GDExtensionBool is_exposed;
 	GDExtensionBool is_runtime;
+	GDExtensionConstStringPtr icon_path;
 	GDExtensionClassSet set_func;
 	GDExtensionClassGet get_func;
 	GDExtensionClassGetPropertyList get_property_list_func;