Extend CSS scroll-direction production to match the WebIDL enum

Author: birtles

Overview.bs

@@ -553,7 +553,7 @@ Canonical order: per grammar
 
 <dfn>&lt;single-animation-timeline></dfn> = auto | scroll([element(<<id-selector>>)[, <<scroll-direction>>[, <<scroll-offset>>[, <<scroll-offset>>[, <<time>>[, <<single-animation-fill-mode>>]]]]]])
 
-<dfn>&lt;scroll-direction></dfn> = auto | horizontal | vertical
+<dfn>&lt;scroll-direction></dfn> = auto | block | inline | horizontal | vertical
 
 <dfn>&lt;scroll-offset></dfn> = <<length>> | <<percentage>> | auto