Add area_stay() signal

[venilark]

If I am playing a game with invincibility frames and dodge too soon, I will hit the enemy's hitbox and area_entered will be called, then the invincibility runs out but I was invincible during area_entered so I remain invincible. To solve this I need an array of entities that collided with the hitbox and a foreach inside _physics_process(). Also I need area_exited to know when I have to remove that area from the array, which means more checks.
This one extra signal would solve this.

Should be the equivalent of Unity's OnTriggerStay.

I'm aware get_overlapping_areas() can also be used but I think this addition would be helpful.

[AThousandShips]

This is likely to be bad for performance, sending signals every frame is expensive