Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
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. 😅
Without this, any relative url()s in the `content` property don't know
what style sheet they are in, which makes them load relative to the
document instead.
