LibWeb: Align Document::document_type() default with the specification

The DOM specification states that: "Unless stated otherwise, a
document’s [...] type is 'xml'".

Previously, calls to `Document::document_type()` were returning the
incorrect value for non-HTML documents.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -648,7 +648,7 @@ private:
     QuirksMode m_quirks_mode { QuirksMode::No };
 
     // https://dom.spec.whatwg.org/#concept-document-type
-    Type m_type { Type::HTML };
+    Type m_type { Type::XML };
 
     bool m_editable { false };