[css-values] Add CanIUse panels

[cvrebert]

Bikeshed has a feature to include CanIUse info panels in specs. (Example; Example commit)

I wonder if Values&Units would consider using this feature? Here are the spec anchors and corresponding CanIUse features for Values&Units:

CSS spec URL	CanIUse feature URL
https://drafts.csswg.org/css-values/#attr-notation	https://caniuse.com/#feat=css3-attr
https://drafts.csswg.org/css-values/#calc	https://caniuse.com/#feat=calc
https://drafts.csswg.org/css-values/#ch	https://caniuse.com/#feat=ch-unit
https://drafts.csswg.org/css-values/#rem	https://caniuse.com/#feat=rem
https://drafts.csswg.org/css-values/#viewport-relative-lengths	https://caniuse.com/#feat=viewport-units

[tabatkins]

Done. ^_^