Before this change, we only parsed fit-content as a standalone keyword,
but CSS-SIZING-3 added it as a function as well. I don't know of
anything else in CSS that is overloaded like this, so it ends up looking
a little awkward in the implementation.
Note that a lot of code had already been prepped for fit-content values
to have an argument, we just weren't parsing it.
Same again, although rotation is more complicated: `rotate`
is "equivalent to" multiple different transform function depending on
its arguments. So we can parse as one of those instead of the full
`rotate3d()`, but then need to handle this when serializing.
The only ways this varies from the `scale()` function is with parsing
and serialization. Parsing stays separate, and serialization is done by
telling `TransformationStyleValue` which property it is, and overriding
its normal `to_string()` code for properties other than `transform`.
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.
