LibWeb/CSS: Add CSSOM types for @layer rules

Depending on usage, `@layer` has two forms, with two different CSSOM
types. One simply lists layer names and the other defines a layer with
its contained rules.

Userland/Libraries/LibWeb/CSS/CSSLayerBlockRule.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSLayerBlockRule.h"
+#include <LibWeb/Bindings/CSSLayerBlockRulePrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/Serialize.h>
+
+namespace Web::CSS {
+
+JS_DEFINE_ALLOCATOR(CSSLayerBlockRule);
+
+JS::NonnullGCPtr<CSSLayerBlockRule> CSSLayerBlockRule::create(JS::Realm& realm, FlyString name, CSSRuleList& rules)
+{
+    return realm.heap().allocate<CSSLayerBlockRule>(realm, realm, move(name), rules);
+}
+
+FlyString CSSLayerBlockRule::next_unique_anonymous_layer_name()
+{
+    static u64 s_anonymous_layer_id = 0;
+    return MUST(String::formatted("#{}", ++s_anonymous_layer_id));
+}
+
+CSSLayerBlockRule::CSSLayerBlockRule(JS::Realm& realm, FlyString name, CSSRuleList& rules)
+    : CSSGroupingRule(realm, rules)
+    , m_name(move(name))
+{
+    if (m_name.is_empty()) {
+        m_name_internal = next_unique_anonymous_layer_name();
+    } else {
+        m_name_internal = m_name;
+    }
+}
+
+void CSSLayerBlockRule::initialize(JS::Realm& realm)
+{
+    Base::initialize(realm);
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSLayerBlockRule);
+}
+
+String CSSLayerBlockRule::serialized() const
+{
+    // AD-HOC: No spec yet, so this is based on the @media serialization algorithm.
+    StringBuilder builder;
+    builder.append("@layer"sv);
+    if (!m_name.is_empty())
+        builder.appendff(" {}", m_name);
+
+    builder.append(" {\n"sv);
+    // AD-HOC: All modern browsers omit the ending newline if there are no CSS rules, so let's do the same.
+    if (css_rules().length() == 0) {
+        builder.append('}');
+        return builder.to_string_without_validation();
+    }
+
+    for (size_t i = 0; i < css_rules().length(); i++) {
+        auto rule = css_rules().item(i);
+        if (i != 0)
+            builder.append("\n"sv);
+        builder.append("  "sv);
+        builder.append(rule->css_text());
+    }
+
+    builder.append("\n}"sv);
+
+    return builder.to_string_without_validation();
+}
+
+FlyString CSSLayerBlockRule::internal_qualified_name(Badge<StyleComputer>) const
+{
+    // TODO: Cache this?
+    auto parent_name = parent_layer_internal_qualified_name();
+    if (parent_name.is_empty())
+        return m_name_internal;
+    return MUST(String::formatted("{}.{}", parent_name, m_name_internal));
+}
+
+}