To support this, how we declare logical property aliases has changed.
Instead of `logical-alias-for` being a list of properties, it's now an
object with a `group` and `mapping`. The group is the name of a logical
property group in LogicalPropertyGroups.json. The mapping is which
side/dimension/corner this property is. Hopefully it's self-explanatory
enough.
The generated code is very much a copy of what was previously in
`StyleComputer::map_logical_alias_to_physical_property_id()`, so there
should be no behaviour change.
