LibWeb: Add customElements.getName

Author: https://github.com/keithamus 🔰
Commit: 8d593bcfeb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/458
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/tcl3

Userland/Libraries/LibWeb/HTML/CustomElements/CustomElementRegistry.idl

@@ -5,6 +5,7 @@
 interface CustomElementRegistry {
     [CEReactions] undefined define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions options = {});
     (CustomElementConstructor or undefined) get(DOMString name);
+    DOMString? getName(CustomElementConstructor constructor);
     Promise<CustomElementConstructor> whenDefined(DOMString name);
     [CEReactions] undefined upgrade(Node root);
 };