These get computed to an equivalent RGBColorStyleValue.
To support this, we now store the computed color-scheme on the
ComputationContext when computing properties that might contain a color.
This has a nice bonus of correcting the css-accent-color test's result.
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.
For us, that's KeywordStyleValue and CustomIdentStyleValue.
CustomIdentStyleValue now has a .cpp file to reduce the number of
includes in its header file.
