Since we now have access to the `AbstractElement` through the
`ComputationContext` we can just set the flag that this element relies
on tree counting functions directly, no need to pass this struct around.
This struct will in the future hold information other than a length
resolution context (e.g. context for tree counting functions) and a
single struct is easier to work with than multiple parameters.
We shouldn't include spread distance when serializing `text-shadow` as
it is not supported unlike `box-shadow` - to achieve this we store
whether this is a text or box shadow within the ShadowStyleValue and
serialize appropriately.
This reverts 0e3487b9ab.
Back when I made that change, I thought we could make our StyleValue
classes match the typed-om definitions directly. However, they have
different requirements. Typed-om types need to be mutable and GCed,
whereas StyleValues are immutable and ideally wouldn't require a JS VM.
While I was already making such a cataclysmic change, I've moved it into
the StyleValues directory, because it *not* being there has bothered me
for a long time. 😅
