Handle scale factor change on web-sys backend (#1690)

* Change web backend `event_handler` to without  'static lifetime

* Refactor web runner and fix ControlFlow::Exit not being sticky

* Impl. scaling change event for web-sys backend

* Improve `dpi` docs regarding the web backend

* Add changes to changelog

* Update features.md

src/platform_impl/web/web_sys/mod.rs

@@ -1,8 +1,10 @@
 mod canvas;
 mod event;
+mod scaling;
 mod timeout;
 
 pub use self::canvas::Canvas;
+pub use self::scaling::ScaleChangeDetector;
 pub use self::timeout::{AnimationFrameRequest, Timeout};
 
 use crate::dpi::{LogicalSize, Size};