### Godot version

4.0 beta 10, also 3.5.1

### System information

Windows 11

### Issue description

Creating a Mainloop inherited from SceneTree makes sense when you want to add a few features to SceneTree instead of starting from scratch. Unfortunately, this doesn't work as well, as if its type is erased.

But inheriting from Mainloop directly (in the example) can work properly.


### Steps to reproduce

1. Create a mainloop extends SceneTree. In this case, I call it `MyGDScriptMainLoop`.

```gdscript
class_name MyGDScriptMainLoop extends SceneTree

func say_hello():
    print("hello")
```

2. Go to project settings, set `application/run/main_loop_type` to `MyGDScriptMainLoop`.
3. Create a test scene and try to call `Engine.get_main_loop().say_hello()`.

```gdscript
extends Node2D


# Called when the node enters the scene tree for the first time.
func _ready() -> void:
    Engine.get_main_loop().say_hello()
    pass # Replace with function body.
```

Then you will see:

![image](https://user-images.githubusercontent.com/93276898/210129049-0250a15f-2f87-49a1-b066-16ef5e695302.png)

And also I found that `Engine.get_main_loop() is MyGDScriptMainLoop` is `false`.

Using GDExtension to do this also failed. But since I'm using godot-rust, I'm not sure if this problem exists with godot-cpp.

### Minimal reproduction project

[bug.zip](https://github.com/godotengine/godot/files/10327002/bug.zip)

static uint64_t quit_after = 0;
static OS::ProcessID editor_pid = 0;
#ifdef TOOLS_ENABLED
static bool found_project = false;
static bool auto_build_solutions = false;
static String debug_server_uri;
#ifndef DISABLE_DEPRECATED
static int converter_max_kb_file = 4 * 1024; 
static int converter_max_line_length = 100000;
#endif 
HashMap<Main::CLIScope, Vector<String>> forwardable_cli_arguments;
#endif
static bool single_threaded_scene = false;
static DisplayServer::WindowMode window_mode = DisplayServer::WINDOW_MODE_WINDOWED;
static DisplayServer::ScreenOrientation window_orientation = DisplayServer::SCREEN_LANDSCAPE;
static DisplayServer::VSyncMode window_vsync_mode = DisplayServer::VSYNC_ENABLED;
static uint32_t window_flags = 0;
static Size2i window_size = Size2i(1152, 648);
static int init_screen = DisplayServer::SCREEN_PRIMARY;
static bool init_fullscreen = false;
static bool init_maximized = false;
static bool init_windowed = false;
static bool init_always_on_top = false;
static bool init_use_custom_pos = false;
static bool init_use_custom_screen = false;
static Vector2 init_custom_pos;
static bool use_debug_profiler = false;
#ifdef DEBUG_ENABLED
static bool debug_collisions = false;
static bool debug_paths = false;