LibWeb: Add stub implementation of HTMLOptionsCollection

This is a subtype of `DOM::HTMLCollection` that only holds
`HTMLOptionElement`s. In this stub implementation only `item`,
`namedItem` and `length`, inherited from HTMLCollection, are exposed.
This is good enough for applications that only read the collection.

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/HTMLOptionsCollection.h>
+
+namespace Web::HTML {
+
+HTMLOptionsCollection::HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter)
+    : DOM::HTMLCollection(root, move(filter))
+{
+}
+
+}

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/HTMLCollection.h>
+
+namespace Web::HTML {
+
+class HTMLOptionsCollection final : public DOM::HTMLCollection {
+public:
+    using WrapperType = Bindings::HTMLOptionsCollectionWrapper;
+
+    static NonnullRefPtr<HTMLOptionsCollection> create(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter)
+    {
+        return adopt_ref(*new HTMLOptionsCollection(root, move(filter)));
+    }
+
+protected:
+    HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter);
+};
+
+}
+
+namespace Web::Bindings {
+
+HTMLOptionsCollectionWrapper* wrap(JS::GlobalObject&, HTML::HTMLOptionsCollection&);
+
+}

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.idl

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+#import <DOM/HTMLCollection.idl>
+#import <HTML/HTMLOptionElement.idl>
+#import <HTML/HTMLOptGroupElement.idl>
+
+[Exposed=Window]
+interface HTMLOptionsCollection : HTMLCollection {
+    // [CEReactions] attribute unsigned long length; // shadows inherited length
+    // [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
+    // [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
+    // [CEReactions] undefined remove(long index);
+    // attribute long selectedIndex;
+};