[mediaqueries-5] Add viewport segment media features

[dlibby-]

Add two new media feature that allow for authors to detect situations where the viewport is logically separated based on the window's configuration with respect to display features. This is designed to allow authors to target foldable devices where the viewport spans multiple displays (logical or physical).

There are features for both the horizontal and vertical directions and are integers so that future form factors (think a tri-fold device) that do not yet exist can be explicitly detected and supported by authors.

Fixes #5621

[dlibby-]

@tabatkins - mind taking a look? Thanks!

[dlibby-]

Friendly ping @frivoal @tabatkins

[dlibby-]

I'm hoping to make progress on these features over this quarter, so will probably merge this shortly unless there are any objections. The additions are relatively small, so I'm comfortable hashing out feedback via follow up issues.