Caniuse: Add compat data for CSS scrollbars spec

[nt1m]

https://drafts.csswg.org/css-scrollbars-1/

The current data on caniuse.com is outdated, and only has the non-standard webkit and MS properties.

Firefox supports the official specification.

[Schweinepriester]

+1

• https://www.smashingmagazine.com/2018/10/tpac-css-working-group-new/#css-scrollbars
• https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Scrollbars

EDIT:

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/64#CSS

CSS Scrollbars has been enabled by default (bug 1492012).

[Fyrd]

https://caniuse.com/#feat=css-scrollbar includes official support, though there's also https://caniuse.com/#feat=mdn-css_properties_scrollbar-color etc. for individual properties.