LibJS: Implement a nearly empty Intl.DateTimeFormat object

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

Note that unlike other Intl objects, the Intl.DateTimeFormat object has
a LibUnicode structure as a base. This is to prevent wild amounts of
code duplication between LibUnicode, Intl.DateTimeFormat, and other
not-yet-defined Intl structures, because there's 12 fields shared
between them.

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

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/NativeFunction.h>
+
+namespace JS::Intl {
+
+class DateTimeFormatConstructor final : public NativeFunction {
+    JS_OBJECT(DateTimeFormatConstructor, NativeFunction);
+
+public:
+    explicit DateTimeFormatConstructor(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~DateTimeFormatConstructor() override = default;
+
+    virtual ThrowCompletionOr<Value> call() override;
+    virtual ThrowCompletionOr<Object*> construct(FunctionObject& new_target) override;
+
+private:
+    virtual bool has_constructor() const override { return true; }
+};
+
+}