LibWeb: Implement DOMStringList

Author: https://github.com/jamierocks
Commit: 75216182c9
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/731
Reviewed-by: https://github.com/AtkinsSJ

Userland/Libraries/LibWeb/HTML/DOMStringList.idl

@@ -0,0 +1,7 @@
+// https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#domstringlist
+[Exposed=(Window,Worker)]
+interface DOMStringList {
+    readonly attribute unsigned long length;
+    getter DOMString? item(unsigned long index);
+    boolean contains(DOMString string);
+};