Add is_scrolling/is_smooth_scrolling util, checking for active scroll action. (#7669)

* Closes #7657
* [x] I have followed the instructions in the PR template

On native this uses a new "touch phase" parameter of the mouse wheel
event to know if a scroll action is done.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/web/events.rs

@@ -831,6 +831,7 @@ fn install_wheel(runner_ref: &WebRunner, target: &EventTarget) -> Result<(), JsV
                 unit,
                 delta,
                 modifiers,
+                phase: egui::TouchPhase::Move,
             }
         };
         let should_stop_propagation = (runner.web_options.should_stop_propagation)(&egui_event);