Add is_instance() helper method to Node

[KoBeWi]

Adds a helper method to check whether a node is an instance. This allows differentiating intentions between checking empty path and checking for an instance (which implementation-wise is the same thing).

I noticed we have lots of repeated "magic" code for checking things like instances, inheritance, ownership etc. Adding helper methods for such cases makes the code more readable, so I plan to make follow-up PRs with more helpers.

[RandomShaper]

This may be one of those cases where a seemingly helpful abstraction actually makes code worse to reason about. In some places we have code like this:

if (!node.get_scene_file_path().is_empty()) {
  // Do something with node.get_scene_file_path().
  // (I know there's a path for sure!)
}

After this PR that'd become this:

if (node.is_instance()) {
  // Do something with node.get_scene_file_path().
  // (Hmm... Is there a path or can it be empty?)
}

Maybe it depends on the level of abstraction the code dealing with the Node is working at. However, having the helper method but not universally allowed or recommended would be not ergonomic.

On the other hand, I also find those checks a bit annoying. So, maybe we could find a stronger proxy? For instance, a rewording of is_instance() to something else that more strongly implies there's a path for sure, or using a different concept,

I guess, since this is the beginning of a stream of similar PRs, this warrants a Discussion label.

[KoBeWi]

After this PR that'd become this:

It wouldn't, I didn't replace all empty path checks, only these that specifically check for instance.
Examples of code that checks only path:

godot/editor/editor_node.cpp

Lines 1879 to 1880 in b9437c3

	if (!scene->get_scene_file_path().is_empty() && _validate_scene_recursive(scene->get_scene_file_path(), scene)) {
	show_accept(TTR("This scene can't be saved because there is a cyclic instance inclusion.\nPlease resolve it and then attempt to save again."), TTR("OK"));

godot/editor/editor_node.cpp

Line 1989 in b9437c3

if (scene && !scene->get_scene_file_path().is_empty()) { // Only autosave if there is a scene and if it has a path.

godot/editor/editor_data.cpp

Lines 861 to 862 in b9437c3

	if (edited_scene[p_idx].root->get_scene_file_path().is_empty()) {
	return TTR("[unsaved]");

godot/editor/editor_interface.cpp

Lines 693 to 698 in b9437c3

	Error EditorInterface::save_scene() {
	if (!get_edited_scene_root()) {
	return ERR_CANT_CREATE;
	}
	if (get_edited_scene_root()->get_scene_file_path().is_empty()) {
	return ERR_CANT_CREATE;

[RandomShaper]

You've been more clever than me, it seems. 😀 Makes sense.