Add CONNECT_DISCARD_EXTRA connection flag

[KoBeWi]

Closes godotengine/godot-proposals#762

Basically the flag makes any excess argument discarded, so that "Too many arguments" error doesn't appear. It works, you can test it with this code:

func _ready() -> void:
	$AnimationPlayer.animation_finished.connect(queue_free, [], CONNECT_DISCARD_EXTRA)

queue_free() isn't called without CONNECT_DISCARD_EXTRA.

HOWEVER, the "Too many arguments" error still appears despite successful call, so I have to fix it. Also the flag isn't available in connection dialog yet.

[Xrayez]

I wonder whether it makes more sense to discard extra arguments by default ("Too few arguments" would still be a valid error obviously).

[KoBeWi]

Superseded by #42683
(although this could theoretically be applied for 3.2, because the other PR is completely incompatible)