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_settings.h

@@ -47,7 +47,6 @@ class EditorSettings : public Resource {
 	_THREAD_SAFE_CLASS_
 
 public:
-	inline static const String PROJECT_EDITOR_SETTINGS_PATH = "res://.godot/editor";
 	struct Plugin {
 		EditorPlugin *instance = nullptr;
 		String path;