Not every user of this requires an `auto` state, but most do.
This has quite a big diff but most of that is mechanical:
LengthPercentageOrAuto has `resolved_or_auto()` instead of `resolved()`,
and `to_px_or_zero()` instead of `to_px()`, to make their output
clearer.
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. 😅
This reverts commit 76daba3069.
We're going to need separate types for the JS-exposed style values, so
it doesn't make sense for us to match their names with our internal
types.
