[css-overflow] Add spec text for the continue variant of line-clamp

[andreubotella]

In #7708 it was resolved that, aside from the existing spec text for line-clamp which fragments away the clamped content, spec text should also be added for another variant which lays out the clamped content but doesn't paint it. This issue is about writing spec text for this variant.

I'm currently working on an implementation of this variant in Chromium, which treats line-clamp as a longhand accepting only the values none | auto | <integer [1,∞]>, and I wrote some initial spec text describing the intent of this implementation.

That spec text is still a WIP, and it replaces the existing line-clamp-related sections rather than modifying or extending them, so the plan is not to merge that as is, but to have a somewhat concrete starting point for a conversation.