Move common keyboard types to iced_core

Also expose them in `iced` through `iced_native` and `iced_web`.

src/keyboard.rs

@@ -0,0 +1,6 @@
+//! Listen and react to keyboard events.
+#[cfg(not(target_arch = "wasm32"))]
+pub use iced_winit::input::keyboard::{KeyCode, ModifiersState};
+
+#[cfg(target_arch = "wasm32")]
+pub use iced_web::keyboard::{KeyCode, ModifiersState};

src/lib.rs

@@ -183,6 +183,7 @@ mod element;
 mod sandbox;
 
 pub mod executor;
+pub mod keyboard;
 pub mod settings;
 pub mod widget;
 pub mod window;