This also allows us to remove the path generation and interpolation
handling for `rect()` and `xywh()` since that occurs after computation
Regressions in clip-path-interpolation-xywh.html are due to improper
simplification of length-percentage mixes where the length is 0px.
Storing these within `LengthPercentage`, `LengthBox`, and `Variant`
over-complicated things.
We also now use the correct `SerializationMode` when serializing `xywh`
and `rect`
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. 😅
