LibWeb: Speed up CSS namespace checking

CSSStyleSheet now caches the CSSNamespaceRule for the default namespace,
which is the only one we currently care about. This saves us from
iterating over its list of rules every time we want to know what that
default namespace is.

The spec dictates that `@namespace` rules are only valid near the start
of a stylesheet, so we also take advantage of that to quit searching
for namespaces as soon as we see a non-import rule.

Also renamed `namespace_filter()` to `default_namespace()` since that's
what it actually returns.

This makes github.com/serenityos/serenity snappy again. :^)

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -225,7 +225,7 @@ Vector<MatchingRule> StyleComputer::filter_namespace_rules(DOM::Element const& e
     Vector<MatchingRule> filtered_rules;
 
     for (auto const& rule : rules) {
-        auto namespace_uri = rule.sheet->namespace_filter();
+        auto namespace_uri = rule.sheet->default_namespace();
         if (namespace_uri.has_value()) {
             if (namespace_uri.value() == element.namespace_uri())
                 filtered_rules.append(rule);