Enable DOM walking rules to run on VirtualNode

There are a number of rules largely focused on walking the DOM, either up to the root, or looking at child nodes. Where these rules can run on the virtual tree, we should have them do this, instead of having them go to the DOM some of the time, and virtual tree other times.

I think the following rules could be migrated to use VirtualNode almost exclusively. We'll need to make an exception for aria-owns.

- [ ] aria-required-children
- [ ] aria-required-parent
- [ ] definition-list
- [ ] dlitem
- [ ] list
- [ ] listitem
- [ ] landmark-banner-is-top-level
- [ ] landmark-complementary-is-top-level
- [ ] landmark-contentinfo-is-top-level
- [ ] landmark-main-is-top-level