LibWeb: Parse declarative shadow DOM template elements

We now honor the shadowrootmode attribute on template elements while
parsing, and instantiate a shadow tree as required by the spec.

(cherry picked from commit 9eb4b91168145def5677cd41057d34ecf369372b)

Userland/Libraries/LibWeb/HTML/HTMLTemplateElement.cpp

@@ -63,4 +63,9 @@ void HTMLTemplateElement::cloned(Node& copy, bool clone_children)
     });
 }
 
+void HTMLTemplateElement::set_template_contents(JS::NonnullGCPtr<DOM::DocumentFragment> contents)
+{
+    m_content = contents;
+}
+
 }