LibJS: Define constructor slots for describing how to read options

This is an editorial change in the ECMA-402 spec. See:
a2beb66

We implement this change by introducing a virtual interface that all
Intl "service" objects must implement. A future patch will make use of
the virtualized RelevantExtensionKeys and ResolutionOptionDescriptors
accessors, and we will need to be able to use those slots from a generic
instance type.

Libraries/LibJS/Runtime/Intl/IntlObject.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Span.h>
+#include <AK/StringView.h>
+#include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/PropertyKey.h>
+
+namespace JS::Intl {
+
+// https://tc39.es/ecma402/#resolution-option-descriptor
+struct ResolutionOptionDescriptor {
+    StringView key;
+    PropertyKey property;
+    OptionType type { OptionType::String };
+    ReadonlySpan<StringView> values {};
+};
+
+class IntlObject : public Object {
+    JS_OBJECT(IntlObject, Object);
+
+public:
+    virtual ReadonlySpan<StringView> relevant_extension_keys() const = 0;
+    virtual ReadonlySpan<ResolutionOptionDescriptor> resolution_option_descriptors(VM&) const = 0;
+
+protected:
+    using Object::Object;
+};
+
+}