LibURL: Promote Host to a proper class

This lets us move a few Host-related functions (like serialization and
checks for what the Host is) into Host instead of having them dotted
around the codebase.

For now, the interface is still very Variant-like, to avoid having to
change quite so much in one go.

Libraries/LibWeb/DOM/Document.cpp

@@ -3194,7 +3194,7 @@ String Document::domain() const
         return String {};
 
     // 3. Return effectiveDomain, serialized.
-    return MUST(URL::Parser::serialize_host(effective_domain.release_value()));
+    return effective_domain->serialize();
 }
 
 void Document::set_domain(String const& domain)