Continuation of #519, including the work of @gleblebedev (thank you!), which has been partially squashed here.

Here are the new changes in this PR:

• Changes the navigation algorithm quite a bit.
• Adds some partially related pre-work.
• Introduces the nav shorthand property.
• Adds :focus-visible pseudo class.
• Makes the invaders sample fully work with keyboard navigation, with :focus-visible to highlight the focus.
• Adds some visual tests where we can test how the algorithm behaves in different situations.
• Makes navigation take precedent in select elements, instead one can press space to open its options.
• Restrains auto navigation to stay within the nearest scroll container.

The algorithm is designed to always pick the nearest intersecting element along a given direction. If it finds none, it will effectively try to select one that may lie outside this intersection area as well. There are different design choices here, I tried to not make something too clever on purpose, it's better that it works predictably for the 95% use case. For other cases it is easier to override it manually.

Currently the nav shorthand is a "box" type shorthand - that is, taking 1 to 4 values and assigns them to the direction up/right/down/left, just like for padding and margin properties. We might want to consider instead to use special keywords like horizontal, vertical, and maybe others.

All feedback is very much welcome. I'll clean up the commits in the end before merging.