Order keys before serialization in (at leat some of the) extracts

[tidoust]

Via w3c/webref#1127 (comment)

It's relatively easy to sort properties for all objects across extracts. Now, while this will ease reviews of diffs, the human readability of some of them might suffer: for instance objects in CSS extracts start with the name property because that seems a useful starting point... We may want to either only sort some of the extracts (e.g. events) or be a bit smarter and start with specific properties in each extract before we move to alphabetic sorting.