LibJS: Implement a nearly empty Intl.ListFormat object

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

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

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS::Intl {
+
+class ListFormat final : public Object {
+    JS_OBJECT(ListFormat, Object);
+
+public:
+    enum class Type {
+        Invalid,
+        Conjunction,
+        Disjunction,
+        Unit,
+    };
+
+    enum class Style {
+        Invalid,
+        Narrow,
+        Short,
+        Long,
+    };
+
+    ListFormat(Object& prototype);
+    virtual ~ListFormat() override = default;
+
+    String const& locale() const { return m_locale; }
+    void set_locale(String locale) { m_locale = move(locale); }
+
+    Type type() const { return m_type; }
+    void set_type(StringView type);
+    StringView type_string() const;
+
+    Style style() const { return m_style; }
+    void set_style(StringView style);
+    StringView style_string() const;
+
+private:
+    String m_locale;                  // [[Locale]]
+    Type m_type { Type::Invalid };    // [[Type]]
+    Style m_style { Style::Invalid }; // [[Style]]
+};
+
+}