[css-align-3] Import auto-margin handling from CSS2

[tabatkins]

The Align spec needs to interact with "auto" margins in a few spots, and currently attempts to do so by monkeypatching the CSS2 algos. This misses some cases, and is generally unclear. We should import all of the auto-margin handling into Align so it can have a single correct definition.

(In particular, we need a clear answer for what to do when you have auto margins and are overconstrained; the current mentions of "overconstrained" in the spec actually assume you have fixed margins, once you dig into their reasoning.)

[fantasai]

I think that we should defer these to the relevant layout module, aside from block layout which we will unfortunately need to handle here for the time being.