LibJS: Implement a nearly empty Intl.PluralRules object

This adds plumbing for the Intl.PluralRules object, constructor, and
prototype.

Userland/Libraries/LibJS/Runtime/Intl/PluralRules.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <LibJS/Runtime/Intl/NumberFormat.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS::Intl {
+
+class PluralRules final : public NumberFormatBase {
+    JS_OBJECT(PluralRules, NumberFormatBase);
+
+public:
+    enum class Type {
+        Cardinal,
+        Ordinal,
+    };
+
+    PluralRules(Object& prototype);
+    virtual ~PluralRules() override = default;
+
+    Type type() const { return m_type; }
+    StringView type_string() const;
+    void set_type(StringView type);
+
+private:
+    Type m_type { Type::Cardinal }; // [[Type]]
+};
+
+}