[resize-observer-1] Update algorithms with new boxes

[atotic]

[resize-observer-1] Update algorithms with new boxes

A large pull request, algorithm redo touched everything.

The main work was modifying algorithms to work with multiple boxes. This work should be complete. I assumed RO is only observing 1 box at a time, as discussed in #3329.

Minor work included:

• fixing some of the spec bugs that were opened by domenic in the old repo.
• switching from contentRect to borderBoxSize in examples, no need to showcase old API.

One last large remaining problem is ResizeObservation. It should not be an idl interface, but a bag of slots and algorithms. I'll do this next week.

I am not very proficient with bikeshed, especially references and slots. There is probably a lot of room for cleanup by an expert.

[atotic]

I've just connected my w3c account to github.