[css-display] Define `legend` as a `<display-outside>` value

[fantasai]

Splitting out from #3094 (comment) and @MatsPalmgren's comment in #3771: legend should ideally be a display value, not its own property.

[fantasai]

[This issue has to figure out whether that's possible from a web-compat perspective, and also, draft up a spec for it based on zcorpan's findings in https://github.com//issues/3094 etc."]