check for overlaps to validate top left

[gavin-ts]

Summary

If top and left keywords are used incorrectly they can cause invalid overlaps which can lead to layout errors.
This adds validation to check that the top and left keywords do not guarantee a layout is impossible.

Details

• check if objects in the same container will definitely overlap based on their top and left keywords and their initial dimensions
• Note: if container objects need to grow past their initial dimensions to fit descendants during layout they can still end up overlapping with to top and left, causing a layout error.