Register and cleanup resource importer singletons in a predictable way

[YuriSizov]

Fixes #80255. Replaces #79981.

I think I have an idea of what's happening. We create all importers in EditorNode, and we assume those will act as singletons (for some of them which implement the pattern). However, there is a possibility for something else to steal the instance, and then be discarded. In this case it looks like the doc generator does that. It has this bit:

if (ClassDB::is_parent_class(name, "ResourceImporter") && name != "EditorImportPlugin" && ClassDB::can_instantiate(name)) {
	import_option = true;
	ResourceImporter *resimp = Object::cast_to<ResourceImporter>(ClassDB::instantiate(name));
	List<ResourceImporter::ImportOption> options;
	resimp->get_import_options("", &options);
	for (int i = 0; i < options.size(); i++) {
		const PropertyInfo &prop = options[i].option;
		properties.push_back(prop);
		import_options_default[prop.name] = options[i].default_value;
	}
	own_properties = properties;
	memdelete(resimp);
}

If this code runs after the EditorNode constructor, it will assume the place of the singleton, and then it will immediately be destroyed, nulling the reference. My proposed solution to this is to add a flag for the constructor so we explicitly decide which instance is going to be added as the singleton instance. I implemented it for all 3 importers that have singletons. And also made sure we don't leak anything on exit for all of them.

Note that the layered texture importer has the singleton, but it's unused as far as I can see.

---

Due to this being a race condition of sorts, it's hard to reproduce reliably, thus making it harder to validate the fix. But I think the logic is solid. When testing note that the doc generator only runs if you don't have a valid docs cache. So make sure to delete it (editor_data/cache/editor_doc_cache.res).

[akien-mga]

Seems like a good option to solve this.

Would like a check from @RandomShaper but he's currently on holiday, so he can review post merge.

[akien-mga]

Thanks!