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)

[stleamist]

Hi @lucdion,

I've made a proposal to exclude subviews from the size calculation without removing them. I'd appreciate it if you could review it! :)

[lucdion]

Just missing changes related to the documentation 🙏

[stleamist]

I'll let you know when the documentation is complete!