prefix private functions with underscore

scripts/globals/fullscreen_manager.gd

@@ -4,10 +4,10 @@ var is_fullscreen: bool = false
 
 func _input(event: InputEvent) -> void:
 	if event.is_action_pressed("toggle_fullscreen"):
-		toggle_fullscreen()
+		_toggle_fullscreen()
 
-func toggle_fullscreen() -> void:
-	is_fullscreen = !is_fullscreen
+func _toggle_fullscreen() -> void:
+	is_fullscreen = not is_fullscreen
 	if is_fullscreen:
 		DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
 	else: