Refactor editor paths validation in EditorPaths and EditorSettings

- EditorSettings: Ensure that `create()` makes a valid singleton.
  Fixes #49179, fixes #49450.
- EditorPaths: Cleanup code, properly set `paths_valid`.
- EditorPaths: Move more paths validation (check, mkdir) from
  EditorSettings for a better separation of concerns.
- EditorPaths: Move EditorFileSystem creation of `.godot/imported`
  next to other paths.

editor/editor_node.cpp

@@ -3728,10 +3728,6 @@ bool EditorNode::is_scene_in_use(const String &p_path) {
 	return false;
 }
 
-void EditorNode::register_editor_paths(bool p_for_project_manager) {
-	EditorPaths::create(p_for_project_manager);
-}
-
 void EditorNode::register_editor_types() {
 	ResourceLoader::set_timestamp_on_load(true);
 	ResourceSaver::set_timestamp_on_save(true);