Add position-4.vim

https://www.w3.org/TR/2025/WD-css-position-4-20250708/

after/syntax/css/position-4.vim

@@ -0,0 +1,2 @@
+syn keyword cssFontProp contained overlay
+syn keyword cssPseudoClassId contained backdrop

test/test.css

@@ -879,6 +879,10 @@ mask {
   inset: auto;
 }
 
+.position-4::backdrop {
+  overlay: auto;
+}
+
 .properties-values-api-1 {
   syntax: auto;
   inherits: auto;