LibWeb: Add HTMLTemplateElement IDL attrs for declarative shadow DOM

Author: https://github.com/awesomekling
Commit: a0e1112209
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/278
Reviewed-by: https://github.com/mattco98
Reviewed-by: https://github.com/nico

Userland/Libraries/LibWeb/HTML/HTMLTemplateElement.idl

@@ -8,5 +8,8 @@ interface HTMLTemplateElement : HTMLElement {
     [HTMLConstructor] constructor();
 
     readonly attribute DocumentFragment content;
-
+    [Reflect=shadowrootmode, Enumerated=ShadowRootMode, CEReactions] attribute DOMString shadowRootMode;
+    [Reflect=shadowrootdelegatesfocus, CEReactions] attribute boolean shadowRootDelegatesFocus;
+    [Reflect=shadowrootclonable, CEReactions] attribute boolean shadowRootClonable;
+    [Reflect=shadowrootserializable, CEReactions] attribute boolean shadowRootSerializable;
 };