Improve autoload

project.godot will be rewritten every time when reopen project because duplicate registration autoload

change anima_plugin.gd

```
func _enter_tree():
	add_autoload_singleton("ANIMA", 'res://addons/anima/core/constants.gd')
```

to 

```
func _enter_tree():
	if not ProjectSettings.has_setting("autoload/ANIMA"):
		add_autoload_singleton("ANIMA", 'res://addons/anima/core/constants.gd')
```