Add `isIncludedInPinLayoutSizeCalculation` property to `Layoutable`

[stleamist]

This PR adds isIncludedInPinLayoutSizeCalculation property to Layoutable.

This can be useful when calculating the size of a view that contains subviews that may disappear based on conditions after the initial load. (similary to UIStackView's arrangedSubviews, or FlexLayout's isIncludedInLayout)