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 introduces the `TextUnderlinePositionStyleValue` class, it is
possible to represent `text-underline-position` as a `StyleValueList`
but would have required ugly workarounds for either serialization or in
`ComputedProperties::text_underline_position`
