Implement non-XR Gamepad discovery and input (#31200)

* Create embedder event to send to constellation

* Handle gamepad message in constellation, send to script thread

* Handle GamepadEvent in script thread and dispatch event to document

* Add missing Clones, fix event

* Add gamepad task source

* Adjust GamepadIndex type, remove unused imports

* Add internal getter for gamepads list

* Update gamepad new methods

* Handle gamepad connect and disconnect events

* Proto will be none, no need for HandleObject

* Initialize buttons and axes to standard mapping

* Adjust update type index types

* Update GamepadButton update function

* Adjust Gamepad mapping comments to match spec, add update logic

* Amend comment

* Update button and axis inputs on Updated event

* Add GilRs as gamepad backend in servoshell

* Add spec links, queue gamepad updates on task source

* ./mach fmt

* Fix comment length

* Split out button init, update spec comments

* Move gamepad event handling from document to global

* Map and normalize axes/button values

* Use std::time for gamepad timestamp

* Adjust gamepad handling in event loop

* Move button press/touch check into map+normalize function

- Small change but is more in line with spec

* ./mach fmt

* Update comment spec links and warning messages

* Doc comments -> regular comments

* Add window event handlers for gamepad connect/disconnect

* Adjust gamepad disconnect behavior

* Add missing TODO's, adjust gamepad/gamepadbutton list methods and formatting

* Update button handling from gilrs, add comments

* Enable gamepad pref during WPT tests and update expectations

* Update WPT expectations in meta-legacy-layout

components/script/dom/gamepadlist.rs

@@ -18,8 +18,6 @@ pub struct GamepadList {
     list: DomRefCell<Vec<Dom<Gamepad>>>,
 }
 
-// TODO: support gamepad discovery
-#[allow(dead_code)]
 impl GamepadList {
     fn new_inherited(list: &[&Gamepad]) -> GamepadList {
         GamepadList {
@@ -46,6 +44,10 @@ impl GamepadList {
             }
         }
     }
+
+    pub fn remove_gamepad(&self, index: usize) {
+        self.list.borrow_mut().remove(index);
+    }
 }
 
 impl GamepadListMethods for GamepadList {