[css-device-adapt] Define viewport-fit in the section of  Viewport <META> element

https://drafts.csswg.org/css-device-adapt/#viewport-meta

Both Chrome and Safari have shipped vireport-fit in meta viewport tag, and Firefox is going to ship it soon ([bug 1574307](https://bugzilla.mozilla.org/show_bug.cgi?id=1574307)).  Unfortunately the CSS device adaptation spec doesn't mention it at all even in non normative.  It would be nice to have a subsection for it in the spec.