LibWeb: Support Document.onreadystatechange

This is just a standard IDL event attribute handler.

Fixes at least one WPT test:
https://wpt.live/html/dom/documents/resource-metadata-management/document-readyState.html

Tests/LibWeb/Text/expected/HTML/onreadystatechange.txt

@@ -0,0 +1 @@
+interactive

Tests/LibWeb/Text/input/HTML/onreadystatechange.html

@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    let readyStates = []
+    document.onreadystatechange = function() {
+        readyStates.push(document.readyState);
+    }
+    test(() => {
+        println(readyStates);
+    });
+</script>

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -5666,4 +5666,14 @@ Document::StepsToFireBeforeunloadResult Document::steps_to_fire_beforeunload(boo
     return { unload_prompt_shown, unload_prompt_canceled };
 }
 
+WebIDL::CallbackType* Document::onreadystatechange()
+{
+    return event_handler_attribute(HTML::EventNames::readystatechange);
+}
+
+void Document::set_onreadystatechange(WebIDL::CallbackType* value)
+{
+    set_event_handler_attribute(HTML::EventNames::readystatechange, value);
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -731,6 +731,9 @@ public:
     };
     StepsToFireBeforeunloadResult steps_to_fire_beforeunload(bool unload_prompt_shown);
 
+    [[nodiscard]] WebIDL::CallbackType* onreadystatechange();
+    void set_onreadystatechange(WebIDL::CallbackType*);
+
 protected:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -149,6 +149,10 @@ interface Document : Node {
     boolean queryCommandState(DOMString commandId);
     boolean queryCommandSupported(DOMString commandId);
     DOMString queryCommandValue(DOMString commandId);
+
+    // special event handler IDL attributes that only apply to Document objects
+    [LegacyLenientThis] attribute EventHandler onreadystatechange;
+    [FIXME] attribute EventHandler onvisibilitychange;
 };
 
 dictionary ElementCreationOptions {