feat(color-contrast): greatly improve performance for very large sites

[straker]

Remove the scroll requirement from color-contrast and use the new performance api of dom.getElementStack (#1842). I also tried to milk the performance of the dom.getElementStack api for all it's worth (using TreeWalker, caching lookups, etc.), but I think it's the best I can get it. Lastly, to get correct color contrast, I needed every node on the page to be part of the 2d grid, even if they weren't in the axe context. So I had to create temporary VirtualNodes for any node that was missing from the virtual tree so I could use them in the stack.

This returns my favorite site https://web.archive.org/web/20190613132353/https://giveawaylisting.com/ in about 5s. Slower than I wanted but way better than never returning. All other sites seem to return in about the same time as the old code.

Looking at the flame chart, you can see that the initial setting up of the 2d grid takes 2s, and then it's very tiny stacks for each of the 50906 nodes. So nothing is taking too long to run, it's just processing that many nodes is the bottleneck and something that probably can't be overcome.

For #1685, the performance went from 23s to just a few seconds to run color-contrast (had to use an archived view again https://web.archive.org/web/20180625151005/https://users.math.msu.edu/users/wongwwy/expository/torsion.html).

Closes issue: #1588, #318, #1358, #1920, #1730, #1685, #1539, #1951

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