Follow changes in WD-css-break-4-20181218

See also: https://www.w3.org/TR/2018/WD-css-break-4-20181218/

after/syntax/css/break-4.vim

@@ -0,0 +1,2 @@
+syn match cssBoxProp contained "\<margin-break\>"
+syn keyword cssFontAttr contained keep

test/test.css

@@ -66,6 +66,11 @@
   display: verso;
 }
 
+.break-4 {
+  margin-break: auto;
+  display: keep;
+}
+
 .cascade-3 {
   all: auto;
   display: initial;