Use "not" instead of the exclamation mark in GDScript files

Also add String casts

2d/finite_state_machine/debug/states_stack_displayer.gd

@@ -7,7 +7,7 @@ func _process(_delta):
 	var numbers = ""
 	var index = 0
 	for state in fsm_node.states_stack:
-		states_names += state.get_name() + "\n"
+		states_names += String(state.get_name()) + "\n"
 		numbers += str(index) + "\n"
 		index += 1
 	$States.text = states_names