Merge KeyEventExtra into KeyEvent (#4029)

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

src/platform/mod.rs

@@ -42,15 +42,5 @@ pub mod run_on_demand;
 ))]
 pub mod pump_events;
 
-#[cfg(any(
-    windows_platform,
-    macos_platform,
-    x11_platform,
-    wayland_platform,
-    orbital_platform,
-    docsrs
-))]
-pub mod modifier_supplement;
-
 #[cfg(any(windows_platform, macos_platform, x11_platform, wayland_platform, docsrs))]
 pub mod scancode;