[css-content] Add new 'content' value that suppress boxes for child elements (formerly known as 'content:none' for elements)

[MatsPalmgren]

Follow-up from #6503 which removed content:none for elements, since it wasn't web-compatible.

I would like a resolution (soon please) on whether to retain that functionality under a different keyword, e.g. children-none, empty or whatever.

CC @emilio

[emilio]

The minutes in #6503 said that we were going to file an issue about it to ask for use cases. So probably it won't be super-soon since we probably want developer feedback...