Add box-4.vim

https://www.w3.org/TR/2020/WD-css-box-4-20200421/

after/syntax/css/box-4.vim

@@ -0,0 +1,2 @@
+syn match cssBoxProp contained "\<margin-trim\>"
+syn match cssBoxAttr contained "\<in-flow\>"

test/test.css

@@ -51,6 +51,11 @@
 	display: auto;
 }
 
+.box-4 {
+	margin-trim: auto;
+	display: in-flow;
+}
+
 .break-3 {
 	display: recto;
 	display: verso;