LibWeb: Implement counter-[increment,reset,set] properties

These control the state of CSS counters.

Parsing code for `reversed(counter-name)` is implemented, but disabled
for now until we are able to resolve values for those.

Userland/Libraries/LibWeb/CSS/StyleValues/CounterDefinitionsStyleValue.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CounterDefinitionsStyleValue.h"
+#include <LibWeb/CSS/Serialize.h>
+
+namespace Web::CSS {
+
+String CounterDefinitionsStyleValue::to_string() const
+{
+    StringBuilder stb;
+
+    for (auto const& counter_definition : m_counter_definitions) {
+        if (!stb.is_empty())
+            stb.append(' ');
+
+        if (counter_definition.is_reversed)
+            stb.appendff("reversed({})", counter_definition.name);
+        else
+            stb.append(counter_definition.name);
+
+        if (counter_definition.value)
+            stb.appendff(" {}", counter_definition.value->to_string());
+    }
+
+    return stb.to_string_without_validation();
+}
+
+bool CounterDefinitionsStyleValue::properties_equal(CounterDefinitionsStyleValue const& other) const
+{
+    if (m_counter_definitions.size() != other.counter_definitions().size())
+        return false;
+
+    for (auto i = 0u; i < m_counter_definitions.size(); i++) {
+        auto const& ours = m_counter_definitions[i];
+        auto const& theirs = other.counter_definitions()[i];
+        if (ours.name != theirs.name || ours.is_reversed != theirs.is_reversed || ours.value != theirs.value)
+            return false;
+    }
+    return true;
+}
+
+}