Add autocompletion for a few EditorInterface methods

[Mickeon]

Related to #86753, #86747, #86758, #86764, #86777, and #86798

This PR adds autocompletion for get_editor_viewport_3d and set_main_screen_editor.

This originally started with the intention of adding autocompleted results to way more of these, but I had to ditch the idea for now. A lot of the remaining ones require file paths, and it would be nice to find good method to use everywhere, without iterating over entire folders every time.

I originally copied and pasted the implementation in SceneTree for change_scene_to_file, but the way this is is done is EXTREMELY inefficient and would lead to plenty of duplicated code.

godot/scene/main/scene_tree.cpp

Lines 1689 to 1716 in 89cc635

	void SceneTree::get_argument_options(const StringName &p_function, int p_idx, List<String> *r_options) const {
	if (p_function == "change_scene_to_file") {
	Ref<DirAccess> dir_access = DirAccess::create(DirAccess::ACCESS_RESOURCES);
	List<String> directories;
	directories.push_back(dir_access->get_current_dir());
	while (!directories.is_empty()) {
	dir_access->change_dir(directories.back()->get());
	directories.pop_back();
	dir_access->list_dir_begin();
	String filename = dir_access->get_next();
	while (!filename.is_empty()) {
	if (filename == "." || filename == "..") {
	filename = dir_access->get_next();
	continue;
	}
	if (dir_access->dir_exists(filename)) {
	directories.push_back(dir_access->get_current_dir().path_join(filename));
	} else if (filename.ends_with(".tscn") || filename.ends_with(".scn")) {
	r_options->push_back("\"" + dir_access->get_current_dir().path_join(filename) + "\"");
	}
	filename = dir_access->get_next();
	}
	}

[AThousandShips]

LGTM!

[akien-mga]

Thanks!