Let input JS event be dispatched by keydown instead of keypress (#37078)

1. Let `input` JS event be dispatched by `keydown` instead of
`keypress`, according to spec
2. Fire `input` event for Backspace and Delete. But do so only when
something is actually deleted

Testing: Manually tested and compared with other browsers.
Fixes: #37051
cc @xiaochengh

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

components/script/dom/document.rs

@@ -2374,6 +2374,9 @@ impl Document {
         let mut cancel_state = event.get_cancel_state();
 
         // https://w3c.github.io/uievents/#keys-cancelable-keys
+        // it MUST prevent the respective beforeinput and input
+        // (and keypress if supported) events from being generated
+        // TODO: keypress should be deprecated and superceded by beforeinput
         if keyboard_event.state == KeyState::Down &&
             is_character_value_key(&(keyboard_event.key)) &&
             !keyboard_event.is_composing &&