Move common keyboard types to iced_core

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

core/src/keyboard.rs

@@ -0,0 +1,6 @@
+//! Reuse basic keyboard types.
+mod key_code;
+mod modifiers_state;
+
+pub use key_code::KeyCode;
+pub use modifiers_state::ModifiersState;