Add css-writing-modes-4.vim

This syntax is for new spec WD-css-writing-modes-4-20171207.

See also: http://www.w3.org/TR/2017/WD-css-writing-modes-4-20171207/

This commit also removes deferred properties and values from
`css-writing-modes-3.vim`.

Author: hail2u

after/syntax/css/css-writing-modes-3.vim

@@ -1,8 +1,7 @@
 syn match cssFontProp contained "\<writing-mode\>"
 syn match cssFontProp contained "\<text-\(orientation\|combine-upright\)\>"
 syn match cssFontProp contained "\<glyph-orientation-vertical\>"
-syn keyword cssFontAttr contained before after mixed upright plaintext
-syn match cssFontAttr contained "\<sideways\(-rl\|-lr\)\=\>"
+syn keyword cssFontAttr contained before after mixed upright plaintext sideways
 syn match cssFontAttr contained "\<isolate\(-override\)\=\>"
 syn match cssFontAttr contained "\<horizontal-tb\>"
 syn match cssFontAttr contained "\<vertical-\(rl\|lr\)\>"

after/syntax/css/css-writing-modes-4.vim

@@ -0,0 +1 @@
+syn match cssFontAttr contained "\<sideways-\(rl\|lr\)\>"

test/test.css

@@ -889,15 +889,18 @@
   glyph-orientation-vertical: upright;
   display: plaintext;
   display: sideways;
-  display: sideways-rl;
-  display: sideways-lr;
   display: isolate;
   display: isolate-override;
   display: horizontal-tb;
   display: vertical-rl;
   display: vertical-lr;
 }
 
+.writing-modes-4 {
+  display: sideways-rl;
+  display: sideways-lr;
+}
+
 .html5,
 picture,
 rb,