css: Add a :dnd pseudoclass

This will be used for drag highlighting.

testsuite/css/parser/pseudo-classes-known.css

@@ -53,3 +53,7 @@
 :checked {
   color: red;
 }
+
+:dnd {
+  color: red;
+}

testsuite/css/parser/pseudo-classes-known.ref.css

@@ -53,3 +53,7 @@
 :checked {
   color: rgb(255,0,0);
 }
+
+:dnd {
+  color: rgb(255,0,0);
+}