Make CSS Typed OM to_string() implementations produce Utf16String
instead of building UTF-8 strings and converting at the JS binding
boundary. Use Utf16StringBuilder for numeric, math, and transform
serialization paths that already feed CSS Typed OM stringification.
Keep the older CSSOM StyleValue serializer on StringBuilder for now, and
convert only where Typed OM source values or parser re-entry still need
the existing UTF-8 representation.
Because we store calculations as a tree of CalculationNodes inside a
CalculatedStyleValue, instead of a tree of StyleValues directly, this
implements a create_calculation_node() method on CSSNumericValue.
CSSMathValue::create_an_internal_representation() then calls
create_calculation_node() on itself, and wraps it in a
CalculatedStyleValue.
Lots of WPT passes again! Some regressions, which are expected: `cursor`
fails a test for the same reason it fails other that set it to some
kind of numeric value: We don't distinguish between "can contain a
number" and "can accept a number by itself". This will affect any
similar properties, but overall this is a big improvement.
