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

[hiikezoe]

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). 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.