LibWeb: Add CustomStateSet IDL type

Author: https://github.com/AtkinsSJ
Commit: e63d81b36e
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5302
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/HTML/CustomElements/CustomStateSet.idl

@@ -0,0 +1,5 @@
+// https://html.spec.whatwg.org/multipage/custom-elements.html#customstateset
+[Exposed=Window]
+interface CustomStateSet {
+    setlike<DOMString>;
+};