Full support for HTML in Canvas

I've been looking at how axe-core stacks up to HTML in canvas, which Google has been pushing. Most things already seem to work reasonably well. There are a few cases that I came across that we'll want to fix:

1. Axe-core incorrectly considers `` fallback content as visible on screen. Curiously, these elements make it to the accessibility tree, and are even focusable. Because of this, most things we need axe-core to do to support HTML in canvas already exists. What's missing is a way to check if an element is rendered through HTML in Canvas. I don't know a direct way to detect if an element is painted. There might be one, but if not we can check `layoutsubtree`, that at least tells us if an element can't be on screen.

2. The `subtreeText` function incorrectly considers canvas to be empty because of `contentTypes?.includes('embedded')`. That doesn't seem to matter, even without `layoutsubtree`.