Many CSS grammars call for us to parse `Foo || Bar` but we don't have a
good way to represent this unless we have a custom style value. Usually
we store the values in a `StyleValueList` but since that can only store
non-null elements we have to manually recheck which elements we have at
which index whenever we use it.
This style value holds a Vector of `RefPtr<StyleValue const>` so that we
can represent null values and know what values are at what index without
manually rechecking.
