Merge KeyEventExtra into KeyEvent (#4029)

To make the fields easier to use, and to allow constructing KeyEvent in
user test code.

src/platform_impl/web/mod.rs

@@ -40,7 +40,6 @@ pub(crate) use cursor::{
 pub(crate) use self::event_loop::{
     ActiveEventLoop, EventLoop, PlatformSpecificEventLoopAttributes,
 };
-pub(crate) use self::keyboard::KeyEventExtra;
 pub(crate) use self::monitor::{
     HasMonitorPermissionFuture, MonitorHandle, MonitorPermissionFuture, OrientationLockFuture,
 };