Adds additional contain features and keys

[vwallen]

Adds a feature for the contain property itself, to include the higher level keywords values none, strict, and content which are either the default (no containment) or aliases for already included keyword values.

The main key css.properties.contain is already included in a keyword feature. I'm not sure of the protocol for moving a key from one feature to another, but if the contain feature is accepted as-is, it should reclaim that key.

Also adding keys to the content-visibility feature which required an update compute_from to maintain consistent support reporting