If the object is about to collide and the collision normal direction is
in here (i.e. the object is moving roughly in this direction), the object won't collide.
Should be a list of unit vectors `LEFT`, `RIGHT`, `UP`, or `DOWN`.
Defaults to `[UP]`, i.e. the object will only be able to pass through when it
is jumping upwards, but will collide when it is moving downwards or sideways.
shouldCollide?(this: GameObj , obj: GameObj , normal: Vec2 ): boolean
A function that determines whether the object should collide.
If present, it overrides the `ignoreSides`; if absent, it is
automatically created from `ignoreSides`.