[css-typed-om] Define CSSPercentishArray

[tidoust]

CSSPercentishArray was introduced in 8d19906 but not yet defined. Related property gets further discussed in #1037.

Lack of definition makes IDL inconsistent as a whole (IDL consistency is checked and enforced in w3c/webref before integration in Web Platform Tests). Adding this issue to track resolution of the inconsistency.

[tidoust]

Issue got addressed by yesterday's update (which dropped CSSPercentishArray): a90bdcc