Fixes crash when loading *.escn resources with gdnative #20141

The issue is that ResourceFormatLoaderText is a singleton. It was created in a faulty way in
ResourceFormatLoaderNativeScript::load
It was created on the stack, which caused the static singleton pointer to be overwritten. This
causes then segmentation faults if the singleton is used later on.

IMO singleton creation needs to made safer to avoid other similar issues in the future.

modules/gdnative/nativescript/nativescript.cpp

@@ -1711,8 +1711,7 @@ void NativeReloadNode::_notification(int p_what) {
 }
 
 RES ResourceFormatLoaderNativeScript::load(const String &p_path, const String &p_original_path, Error *r_error) {
-	ResourceFormatLoaderText rsflt;
-	return rsflt.load(p_path, p_original_path, r_error);
+	return ResourceFormatLoaderText::singleton->load(p_path, p_original_path, r_error);
 }
 
 void ResourceFormatLoaderNativeScript::get_recognized_extensions(List<String> *p_extensions) const {