Add optional names to tweens for debugging purposes

[Meorge]

Closes godotengine/godot-proposals#12712 .

This PR adds the set_name() and get_name() methods to Tween. When a Tween is given a name, error messages involving that Tween will display the name. This will help users diagnose which Tween within a scene is experiencing issues, if they have a tween-heavy workflow (i.e., animating lots of things using tweens simultaneously).

Example:

var unnamed_tween = create_tween()
unnamed_tween.tween_callback(func(): print("This tween is looping forever"))
unnamed_tween.set_loops(-1)
# ERROR: Infinite loop detected. Check set_loops() description for more info.

var named_tween = create_tween()
named_tween.set_name("SlideDown")
named_tween.tween_callback(func(): print("This tween is looping forever"))
named_tween.set_loops(-1)
# ERROR: Infinite loop detected in Tween "SlideDown". Check set_loops() description for more info.

[Kaleb-Reid]

Could the name be passed as an optional argument to create_tween()? Another thought is that set_name() could return the same Tween so that you could call create_tween().set_name().set_loops()...