Improve extracts for legacy and aliases CSS properties

[tidoust]

See discussion in w3c/webref#115 (comment).

Goal is to make sure that all CSS properties that appear in extracts have some value definition, somehow, be it through a prose key.

Currently, Reffy just manages to extract the property name for these legacy properties. It would be good if it could be more specific. For instance, for -webkit- properties that are aliases, link to the alias, and/or add a prose key that gives some info about the property.