Adds support for `sibling-index()` and `sibling-count()` when parsing
`<number>` and `<integer>`. This is achieved by a new
`TreeCountingFunctionStyleValue` class which is converted within
`absolutized` to `NumberStyleValue` and `IntegerStyleValue` respectively
There are still a few kinks to work out in order to support these
everywhere, namely:
- There are some `StyleValue`s which aren't absolutized (i.e. those
which are stored within another `StyleValue` without an
`absolutize()` method.
- We don't have a way to represent this new `StyleValue` within
`{Number,Integer}OrCalculated`. This would be fixed if we were to
instead just use the `StyleValue` classes until style computation at
which time they would be absolutized into their respective
primitives (double, i64, etc) bypassing the need for *OrCalculated
entirely.
This excludes `step-end` and `step-start` which are expected to be
converted to the equivalent function at parse time.
We are expected to serialize these as the explicit keywords - previously
we would parse as `EasingStyleValue` and serialize equivalent functions
as the keywords. This caused issues as we would incorrectly serialize
even explicit functions as the keyword.
This also allows us to move the magic easing functions to
`EasingFunction` rather than `EasingStyleValue` which is a bit tidier
Canonicalization can require information that is only known after
compute time (i.e. resolved relative lengths within calcs).
This also allows us to get rid of the `had_explicit_input` flag and just
rely on whether Optional has a value
