Cleanup inconsistent convention of `Rect` fields

[Weibye]

The fields of Rect should either be main_start, main_end, cross_start, cross_end or left, right, top, bottom but not a mix of the two.

Most likely the latter

Originally posted by @Weibye in https://github.com/DioxusLabs/taffy/issues/118#issuecomment-1153124824

[alice-i-cecile]

I think the latter is preferred here; this will make it much easier to reuse across layout algorithms.