[ASScrollNode] Add .scrollDirection property so that internal content sizing can be easily "unlimited" in the direction of scrolling.

[appleguy]

No description provided.

[rurza]

This fixes issue when setting automaticallyManagesContentSize didn't work as expected.
I tried with different sizes for preferredSize, implementation of nodeConstrainedSize in VC. Nothing.
With this PR just set scrollDirection.
I think that setting scrollDirection should set automaticallyManagesContentSize though.

[davertay]

This works for my example where I have an ASScrollNode with a simple vertical ASStackLayoutSpec containing nodes all set to 100% width: https://github.com/davertay/asdkScrollExample

Without this change the contentSize was always being set to the view bounds, even when overriding nodeConstrainedSize in the ASViewController.

Note that this probably also fixes #2851

[garrettmoon]

Does the setter and getter need to be defined for this so that they can hold the instance lock?

Separately, scrollDirection is already defined on ASScrollView as well as scrollableDirections. I think the scrollDirection you're providing here is akin to scrollableDirections on ASScrollView. Any way to reconcile all of this?

[garrettmoon]

Looks good to me!

[appleguy]

Discussed in Slack. Thanks for the reviews everyone!