Expose calculated itemSpacing when using SPACE_BETWEEN aligned items

[jamessimo]

When a Frame/Node has an auto layout that is using SPACE_BETWEEN aka auto mode. I expect the outputted "itemSpacing" to be that value as it's calculated. However no matter what it only shows the user defined value.

Perhaps add "calculatedItemSpacing" or a function to getItemSpacing() so 3rd party apps can get the correct spacing of items that are in Auto Layouts.