LibJS: Implement a nearly empty Intl.RelativeTimeFormat object

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

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

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Array.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibUnicode/Locale.h>
+
+namespace JS::Intl {
+
+class RelativeTimeFormat final : public Object {
+    JS_OBJECT(RelativeTimeFormat, Object);
+
+public:
+    enum class Numeric {
+        Always,
+        Auto,
+    };
+
+    static constexpr auto relevant_extension_keys()
+    {
+        // 17.3.3 Internal slots, https://tc39.es/ecma402/#sec-Intl.RelativeTimeFormat-internal-slots
+        // The value of the [[RelevantExtensionKeys]] internal slot is « "nu" ».
+        return AK::Array { "nu"sv };
+    }
+
+    RelativeTimeFormat(Object& prototype);
+    virtual ~RelativeTimeFormat() override = default;
+
+    String const& locale() const { return m_locale; }
+    void set_locale(String locale) { m_locale = move(locale); }
+
+    String const& data_locale() const { return m_data_locale; }
+    void set_data_locale(String data_locale) { m_data_locale = move(data_locale); }
+
+    String const& numbering_system() const { return m_numbering_system; }
+    void set_numbering_system(String numbering_system) { m_numbering_system = move(numbering_system); }
+
+    Unicode::Style style() const { return m_style; }
+    void set_style(StringView style) { m_style = Unicode::style_from_string(style); }
+    StringView style_string() const { return Unicode::style_to_string(m_style); }
+
+    Numeric numeric() const { return m_numeric; }
+    void set_numeric(StringView numeric);
+    StringView numeric_string() const;
+
+    NumberFormat* number_format() const { return m_number_format; }
+    void set_number_format(NumberFormat* number_format) { m_number_format = number_format; }
+
+private:
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    String m_locale;                                 // [[Locale]]
+    String m_data_locale;                            // [[DataLocale]]
+    String m_numbering_system;                       // [[NumberingSystem]]
+    Unicode::Style m_style { Unicode::Style::Long }; // [[Style]]
+    Numeric m_numeric { Numeric::Always };           // [[Numeric]]
+    NumberFormat* m_number_format { nullptr };       // [[NumberFormat]]
+};
+
+}