LibWeb: Fire input event on user interaction with input element buttons

An input event is now fired when the step up or step down button of an
input element of type number is clicked.

This ensures that any associated <output> element is updated when these
buttons are clicked.

Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -814,6 +814,7 @@ void HTMLInputElement::create_text_input_shadow_tree()
         auto up_callback_function = JS::NativeFunction::create(
             realm(), [this](JS::VM&) {
                 MUST(step_up());
+                user_interaction_did_change_input_value();
                 return JS::js_undefined();
             },
             0, "", &realm());
@@ -832,6 +833,7 @@ void HTMLInputElement::create_text_input_shadow_tree()
         auto down_callback_function = JS::NativeFunction::create(
             realm(), [this](JS::VM&) {
                 MUST(step_down());
+                user_interaction_did_change_input_value();
                 return JS::js_undefined();
             },
             0, "", &realm());