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/DateTimeFormat.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Intl/DateTimeFormat.h>
+
+namespace JS::Intl {
+
+Vector<StringView> const& DateTimeFormat::relevant_extension_keys()
+{
+    // 11.3.3 Internal slots, https://tc39.es/ecma402/#sec-intl.datetimeformat-internal-slots
+    // The value of the [[RelevantExtensionKeys]] internal slot is « "ca", "hc", "nu" ».
+    static Vector<StringView> relevant_extension_keys { "ca"sv, "hc"sv, "nu"sv };
+    return relevant_extension_keys;
+}
+
+// 11 DateTimeFormat Objects, https://tc39.es/ecma402/#datetimeformat-objects
+DateTimeFormat::DateTimeFormat(Object& prototype)
+    : Object(prototype)
+{
+}
+
+DateTimeFormat::Style DateTimeFormat::style_from_string(StringView style)
+{
+    if (style == "full"sv)
+        return Style::Full;
+    if (style == "long"sv)
+        return Style::Long;
+    if (style == "medium"sv)
+        return Style::Medium;
+    if (style == "short"sv)
+        return Style::Short;
+    VERIFY_NOT_REACHED();
+}
+
+StringView DateTimeFormat::style_to_string(Style style)
+{
+    switch (style) {
+    case Style::Full:
+        return "full"sv;
+    case Style::Long:
+        return "long"sv;
+    case Style::Medium:
+        return "medium"sv;
+    case Style::Short:
+        return "short"sv;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+}