* Add snapshot tests for interitem and interline spacings of stack spec
* Improve comment
* Make sure item spacings are properly considered and reset on new lines, snapshot tests included
* ASStackUnpositionedLayout: Take spacing into account when laying out a wrapped stack.
* ASStackLayoutSpec: Add the lineSpacing property.
* Update CHANGELOG.md.
- Avoid returning a NaN flexShrinkAdjustment.
- Bail early (instead of assert) if a line has a positive `flexFactorSum` but then no flexible item can be found via flex adjustments, for example because the flexible items have zero size.
- Continue flexing other lines when one line is inflexible.
- Add unit test for the first 2 fixes.
