Generalize toolbox positioning to support relative-offset context elements

[gibson042]

cf. https://github.com/tc39/ecmarkup/pull/552/files#r1312124976

[michaelficarra]

Let's hold off on this until we get approval on the strategy for #552.

[gibson042]

Why? Assuming static positioning for so many elements is unnecessarily brittle.

[michaelficarra]

I just don't want to introduce the complexity until it buys us something.

[gibson042]

Well, I'd like to adopt #552 with a local stylesheet but doing so breaks the toolbox positioning without something like this. I can address that locally as well, but the code here is strictly a generalization—when el.offsetParent itself has no offsetParent (as is the case for relevant elements with the current stylesheet), then getAbsoluteOffset(el, 'offsetTop') is equivalent to the el.offsetTop that it's replacing, and likewise for offsetLeft. It only has an effect when that assumption doesn't hold, in which case the effect is to fix what would otherwise be a bug.