The `transform` property supports transform functions that sometimes
need their `calc(percentage)` values to be converted to a number instead
of a length. Currently this only applies to the `scale*` family of
functions, which are marked as such in `TransformFunctions.json`.
We were not consistently applying the `NumberPercentage` type to these
functions though, and in addition, any `NumberPercentage` value would
not consider calculated values.
To be properly compatible with calc(), the resolved() methods all need:
- A length resolution context
- To return an Optional, as the calculation might not be resolvable
A bonus of this is that we can get rid of the overloads of `resolved()`
as they now all behave the same way.
A downside is a scattering of `value_or()` wherever these are used. It
might be the case that all unresolvable calculations have been rejected
before this point, but I'm not confident, and so I'll leave it like
this for now.
