LibWeb: Implement indexed property support for HTML::Storage

We only supported named properties on Storage, and as a result
`localStorage[0]` would be disconnected from the Storage's backing map.

Fixes at least 20 subtests in WPT in /webstorage.

Tests/LibWeb/Text/expected/localStorage.txt

@@ -0,0 +1,6 @@
+value
+value
+other
+other
+foo
+foo

Tests/LibWeb/Text/input/localStorage.html

@@ -0,0 +1,17 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        localStorage["test"] = "value";
+        println(localStorage["test"]);
+        println(localStorage.getItem("test"));
+
+        localStorage.setItem("test", "other");
+        println(localStorage["test"]);
+        println(localStorage.getItem("test"));
+
+        // Ensure indexed properties work
+        localStorage[0] = "foo";
+        println(localStorage[0]);
+        println(localStorage.getItem("0"));
+    });
+</script>

Userland/Libraries/LibWeb/HTML/Storage.cpp

@@ -23,10 +23,13 @@ Storage::Storage(JS::Realm& realm)
     : Bindings::PlatformObject(realm)
 {
     m_legacy_platform_object_flags = LegacyPlatformObjectFlags {
+        .supports_indexed_properties = true,
         .supports_named_properties = true,
+        .has_indexed_property_setter = true,
         .has_named_property_setter = true,
         .has_named_property_deleter = true,
         .has_legacy_override_built_ins_interface_extended_attribute = true,
+        .indexed_property_setter_has_identifier = true,
         .named_property_setter_has_identifier = true,
         .named_property_deleter_has_identifier = true,
     };
@@ -168,6 +171,13 @@ Vector<FlyString> Storage::supported_property_names() const
     return names;
 }
 
+Optional<JS::Value> Storage::item_value(size_t index) const
+{
+    // Handle index as a string since that's our key type
+    auto key = String::number(index);
+    return named_item_value(key);
+}
+
 JS::Value Storage::named_item_value(FlyString const& name) const
 {
     auto value = get_item(name);
@@ -182,6 +192,13 @@ WebIDL::ExceptionOr<Bindings::PlatformObject::DidDeletionFail> Storage::delete_v
     return DidDeletionFail::NotRelevant;
 }
 
+WebIDL::ExceptionOr<void> Storage::set_value_of_indexed_property(u32 index, JS::Value unconverted_value)
+{
+    // Handle index as a string since that's our key type
+    auto key = String::number(index);
+    return set_value_of_named_property(key, unconverted_value);
+}
+
 WebIDL::ExceptionOr<void> Storage::set_value_of_named_property(String const& key, JS::Value unconverted_value)
 {
     // NOTE: Since PlatformObject does not know the type of value, we must convert it ourselves.

Userland/Libraries/LibWeb/HTML/Storage.h

@@ -38,9 +38,11 @@ private:
     virtual void initialize(JS::Realm&) override;
 
     // ^PlatformObject
+    virtual Optional<JS::Value> item_value(size_t index) const override;
     virtual JS::Value named_item_value(FlyString const&) const override;
     virtual WebIDL::ExceptionOr<DidDeletionFail> delete_value(String const&) override;
     virtual Vector<FlyString> supported_property_names() const override;
+    virtual WebIDL::ExceptionOr<void> set_value_of_indexed_property(u32, JS::Value) override;
     virtual WebIDL::ExceptionOr<void> set_value_of_named_property(String const& key, JS::Value value) override;
 
     void reorder();