Yes, the name is silly, but it's a StyleValue for list-style, so...
yeah. :^)
Since `list-style-type` and `list-style-image` can both have `none` as a
value, and can appear in any order, we have to handle it separately, and
then assign either or both of those to `none` depending on how many
`none`s there are, and whether those sub-properties already have values.
Added some extra test cases to lists.html to cover list-style-image and
list-style-position parts of the list-style shorthand, and the `none`
values.