fix(create-grid): correctly compute stack order for non-positioned stacking contexts

[straker]

While working on the code, I realized that the spec said that if any element that creates a stacking context it would remove the fake stacks created from float or positioned elements that use auto or 0 z-index. So I was able to merge those functions into a single if statement. I also simplified the loop to remove fake stacks so it only needs to look through the array and splice once (since any new stack removes all previous fake stacks).

Closes: #3929

[WilcoFiers]

1. I tried to refactor this function a bit, in order to help myself understand it better, please use that: https://github.com/dequelabs/axe-core/pull/3932/files

2. I think we should move getStackingOrder into its own file. It's only tangentially related to creating the grid. I think separating these two could make things a little clearer.

Overall, I can't seem to keep this all straight in my head. I hope I didn't miss anything, but I'm not sure.

[straker]

I think we should move getStackingOrder into its own file. It's only tangentially related to creating the grid. I think separating these two could make things a little clearer.

I don't think we should. The reason being that if we did we'd have to add tests for it, which means testing stacking context arrays. Those would be a pain to maintain as doing something like we just did (refactoring magic numbers, adding a new stack where one wasn't before) would break all the tests. I'd much rather test actual stack results like we are doing in getElementStack since those aren't fragile and won't break when we update stacking numbers.

Resolved

[WilcoFiers]

Would have preferred to see createStackingOrder move into its own file, but