add main menu and scene manager

core/scene_manager/scene_manager.tscn

@@ -0,0 +1,44 @@
+[gd_scene format=3 uid="uid://bw8hwkw55j675"]
+
+[ext_resource type="Script" uid="uid://bf1f2kp8ghtei" path="res://core/scene_manager/scene_manager.gd" id="1_7resu"]
+[ext_resource type="Resource" uid="uid://dr612tmciq8pg" path="res://core/scene_manager/scene_config.tres" id="2_vtdwt"]
+
+[node name="SceneManager" type="CanvasLayer" unique_id=399843239]
+layer = 100
+script = ExtResource("1_7resu")
+config = ExtResource("2_vtdwt")
+
+[node name="ColorRect" type="ColorRect" parent="." unique_id=1075660170]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+color = Color(0, 0, 0, 0)
+
+[node name="LoadingScreen" type="Control" parent="." unique_id=1075176512]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+
+[node name="Label" type="Label" parent="LoadingScreen" unique_id=887425331]
+layout_mode = 1
+anchors_preset = 3
+anchor_left = 1.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+offset_left = -169.0
+offset_top = -65.0
+offset_right = -20.0
+offset_bottom = -20.0
+grow_horizontal = 0
+grow_vertical = 0
+theme_override_constants/outline_size = 24
+theme_override_font_sizes/font_size = 32
+text = "Loading..."