Improve Olivero hamburger menu animation performance

Thanks for the patch. As part of a triage I am marking this a task because performance bugs imply backing data, which is not shown, and even then, must meet certain benchmarks.

Fixed CCF for #2.
Please review.

If we slow down the animation, we can see that animating only the transform property results in the menu icon jumping up and down.

Looking at the code, we're also changing the inset-block-start property, so we need to either allow the transition to apply to that property as well, or account for the vertical movement in the transformation (which would be more performant).

&:before {
    inset-block-start: 0;
    transform: rotate(-45deg);
  }

  &:after {
    inset-block-start: 0;
    transform: rotate(45deg);
  }

Not to be super nitpicky, but we still have some jumpiness in here. I think this is from the center bar being toggled between border-top: 3px solid var(...) and border-top: 0. We should override just the color of that border so that it doesn't cause layout shift. It might be easier to see if you apply overflow: hidden to the HTML element to prevent the scrollbar from shifting the right-aligned elements.

One other thing I hadn't considered until right now is the hamburger icon in the sticky desktop header. We should make sure that one works as identically as possible.

I was still seeing some issues in #9. I think I've addressed them now in this patch.

Fixed CCF for #12.
Please review.

Patch #14 is passing tests. It may periodically fail unrelated CKEditor5 tests. RTBC per #15.

• I tried several modifications of this approach just to see if I liked it better or it saved LOC or overhead. Nothing I changed was an improvement, so I'm pleased with what has been provided.
• I ran some performance analysis tools comparing HEAD vs what is in the patch, and this is ~40% less demanding, so a really nice clear improvement. I tried a comparison replacing the use of translateY here with translate3d and there was not enough of a difference to suggest a switch would be benefifical. I ran a few profiles and if anything translate3d was a little more demanding, but even then it was by a microtrivial amount

Tthanks @andy-blum for hopping on a Zoom to walk me through this. This seemed pretty complex but a brief explanation revealed this to be a pretty straghtforward and thoughtful fix.

I'm going to run tests on 10.0.x just in case there are any build tool differences before backporting.